Agroforestry for Natural Resource Management

Agroforestry for natural resource management Ian Nuberg • Brendan George • Rowan Reid (Editors) Agroforestry for nat...

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Agroforestry

for natural resource management

Ian Nuberg • Brendan George • Rowan Reid (Editors)

Agroforestry for natural resource management

Agroforestry for natural resource management

)AN.UBERGs"RENDAN'EORGEs2OWAN2EID %DITORS

© Ian Nuberg, Brendan George and Rowan Reid 2009 All rights reserved. Except under the conditions described in the Australian Copyright Act 1968 and subsequent amendments, no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, duplicating or otherwise, without the prior permission of the copyright owner. Contact CSIRO Publishing for all permission requests. National Library of Australia Cataloguing-in-Publication entry Agroforestry for natural resource management/edited by Ian Nuberg, Brendan George and Rowan Reid. 9780643092242 (pbk.) Includes index. Bibliography. Agroforestry – Australia. Agroforestry – Australia – Environmental aspects. Agroforestry – Australia – Economic aspects. Sustainable agriculture – Australia. Natural resources – Management – Australia. Nuberg, Ian. George, Brendan Hugh. Reid, Rowan. 634.994 Published by CSIRO PUBLISHING 150 Oxford Street (PO Box 1139) Collingwood VIC 3066 Australia Telephone: Local call: Fax: Email: Web site:

+61 3 9662 7666 1300 788 000 (Australia only) +61 3 9662 7555 [email protected] www.publish.csiro.au

Front cover: Trees integrated into the farming landscape. Noel and Kim Passalaqua’s farm (Jayfields Nursery) near Holbrook. Photo by R. Reid. Back cover (left to right): Photo by B. George; Photo © NSW Department of Primary Industries, Photographer David Barnes; Photo by I. Nuberg. Set in 10.5/12.5 Adobe Minion Edited by Adrienne de Kretser Cover and text design by James Kelly Typeset by Desktop Concepts Pty Ltd, Melbourne Printed in Australia by Ligare The book has been printed on paper certified by the Programme for the Endorsement of Forest Chain of Custody (PEFC). PEFC is committed to sustainable forest management through third party forest certification of responsibly managed forests. CSIRO PUBLISHING publishes and distributes scientific, technical and health science books, magazines and journals from Australia to a worldwide audience and conducts these activities autonomously from the research activities of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The views expressed in this publication are those of the author(s) and do not necessarily represent those of, and should not be attributed to, the publisher or CSIRO.

Contents Contributors Acknowledgements 1

Agroforestry as integrated natural resource management

x xi 1

Ian Nuberg, Rowan Reid and Brendan George Integrated NRM: a time of great change for the better What is agroforestry? Agroforestry in Australia Agroforestry for integrated NRM Design and planning processes in agroforestry Summary and invitation to this book References

Part I 2

Environmental function of trees in the landscape Agroforestry and the functional mimicry of natural ecosystems

1 1 5 10 11 14 18

21 23

EC Lefroy Introduction Implications of the mimic concept Perenniality and the trade-off between production and persistence Competition rules in early-stage succession Managing perenniality and competition Conclusion Acknowledgements References 3

Using trees to manage local and regional water balances

23 25 28 30 31 32 33 33 37

Keith Smettem and Richard Harper Introduction How trees use water Groundwater flow systems Ecological optimality and the groundwater balance Conclusion References 4

Agroforestry for the management of water, salt and agricultural diffuse source pollutants

37 38 42 43 48 48 53

Tim Ellis and Albert van Dijk Hillslope processes Agroforestry plantings as sinks for water and pollutants Guiding principles for designing plantings to trap sediments Catchment water balance and stream flow Impacts of agroforestry on catchment water balance and stream flow Impacts of agroforestry on stream salinity Guiding principles and modelling tools for catchment-scale planning

53 55 56 59 60 60 63

vi

Agroforestry for Natural Resource Management

Conclusion References 5

Trees protecting dryland crops and soil

64 65 69

Ian Nuberg and Michael Bennell Introduction Early understanding of the value of windbreaks Physical damage processes Australian National Windbreaks Program Protecting crops from physical damage Cost–benefit analysis of windbreaks Conclusion References 6

Biodiversity and habitat enhancement

69 69 73 76 80 81 82 83 87

David Salt and David Freudenberger Introduction Defining biodiversity Enhancing habitat Modifying agroforestry to enhance biodiversity Applying theory to specific agroforestry situations Agroforestry’s role in protecting biodiversity References and further reading 7

Environmental risk in agroforestry

87 87 89 89 98 102 104 107

Margaret Byrne, Lynley Stone and Melissa Millar Introduction Weed invasion Genetic contamination Assessing the risk Managing the risk Conclusion References 8

Landscape aesthetics and agroforestry

107 108 110 115 120 122 122 127

Grant Revell Introduction Landscape assessment models for agroforestry Landscape management planning and design guidelines New projects for integrated agroforestry lead land management Conclusion References

Part II 9

Productive function of trees in the landscape Wood as a farm product

127 129 131 134 139 141

143 145

Rowan Reid Introduction Hardwoods and softwoods

145 145

Contents

Tree growth and wood production Log value and prices Principles of silviculture References 10

Growing high-quality sawlogs

146 152 154 158 161

Rowan Reid Target logs for sawn timbers and veneers Learning from nature: how native forests grow wood Silvicultural regimes Practical guides for managing competition Pruning for clearwood sawlogs How to prune Silviculture for multiple values References 11

Farm firewood production

161 163 167 169 172 176 179 180 183

Peter Bulman and Ian Nuberg Introduction Favourable environment for a firewood industry Plantation firewood species Firewood silviculture Firewood harvesting and processing systems Viability of growing firewood Role of firewood production in natural resource management Acknowledgements References and further reading 12

Pulpwood production

183 183 186 187 191 192 195 196 196 199

Richard Harper, Keith Smettem, Rowan Reid, Andrew Callister, John McGrath and Paul Brennan Introduction Evolution of the industry Contractual arrangements Agroforestry using pulpwood species Potential negative impacts of plantations Market prospects Site and species selection Soils and landscapes Yield prediction Site and stand management Conclusion Acknowledgements References 13

Trees in grazing systems

199 200 201 202 206 206 208 209 212 212 216 216 216 219

Rowan Reid Introduction Pasture production near trees

219 220

vii

viii


Natural pest control Grazing near trees Stock shade and shelter Trees for shelter Integrating trees and grazing at the farm level References 14

Saltbush for forage production on saltland

223 223 225 227 230 235 239

EG Barrett-Lennard and HC Norman Introduction Saltland in Australia Salinity, waterlogging and inundation Matching sites to pastures Animal nutrition Profitability Water use Establishing saltbush Conclusion Acknowledgements References 15

Productive use and rehabilitation of saline land using trees

239 239 241 243 244 245 245 246 247 247 247 251

Nico Marcar Introduction Extent and nature of dryland salinity Genotypic differences in response to salt and waterlogging Minimising risks to growth Economic considerations Prospects for saline land rehabilitation Conclusion Acknowledgements References 16

Integrated production systems

251 251 253 256 260 261 263 263 263 267

John Bartle Introduction Why mallee? Mallee industry design and development Wider application of the mallee model New species, new products and new industries: FloraSearch Conclusion References

Part III 17

Implementation of agroforestry Financial and economic evaluation of agroforestry

267 268 269 273 277 278 278

281 283

David Thompson and Brendan George Introduction Private and social objectives

283 285

Contents

Financial investment analysis Discounting Treatment of inflation: real vs nominal discount rates Investment criteria Uncertainty and sensitivity analysis Economic analysis: the broader social analysis Conclusion References 18

Enhancing the environmental benefits of agroforestry through government policy mechanisms

286 290 292 293 298 300 305 306 309

David J Pannell Introduction Economic rationale for policy intervention Causes of market failure Net benefit test for market failure Evidence for and against market failure related to agroforestry Types of policy intervention Who should pay? Conclusion References 19

Adoption of agroforestry in Australia

309 310 310 314 315 318 318 319 320 323

Digby Race Introduction Growth of farm forestry in Australia Australian policy context Farm-based forestry: contention and agreement Interest in agroforestry Socioeconomics of agroforestry in the wheat-sheep zone Which landholders might adopt agroforestry Supporting agroforesty adoption: putting trees on Australian farms International experiences and lessons Conclusion References

Index

323 324 325 326 327 327 329 331 335 335 336

341

ix

Contributors

Ed Barrett-Lennard: Principal Research Officer, Department of Agriculture, Western Australia.

Hayley Norman: Research Scientist, CSIRO Livestock Industries, Perth, Western Australia.

John Bartle: Principal Research Scientist, Revegetation Systems Unit, Department of Environment and Conservation, Western Australia.

Ian Nuberg: Senior Lecturer in Agriculture and Animal Science, School of Agriculture, Food and Wine, University of Adelaide.

Mike Bennell: Principal Researcher, Department of Water, Land and Biodiversity Conservation, South Australia.

David Pannell: Professor, School of Agricultural and Resource Economics, University of Western Australia. ARC Federation Fellow.

Paul Brennan: Manager, Sustainable Landscapes, Forest Products Commission, Western Australia.

Digby Race: Principal Researcher, Institute of Land, Water and Society, Charles Sturt University, Albury, New South Wales.

Peter Bulman: Consultancy ‘Farm Tree Systems’ and Executive Officer Mt Lofty Ranges Private Forestry, South Australia. Margaret Byrne: Principal Research Scientist, Department of Environment and Conservation, Western Australia.

Rowan Reid: Senior Lecturer in Farm Trees and Agroforestry, Department of Forest and Ecosystem Science, University of Melbourne.

Andrew Callister: R&D Manager, ITC Forestry Ltd, Albany, Western Australia.

Grant Revell: Senior Lecturer in Landscape Architecture and Associate Dean, Faculty of Architecture, Landscape and Visual Arts, University of Western Australia.

Tim Ellis: Research Scientist, Water Quality and Environmental Flows. CSIRO Land & Water, Canberra.

Daryll Richardson: Director of Agave Education, Education Manager for Future Farm Industries Cooperative Research Centre.

David Freudenberger: Principal Research Scientist, CSIRO Sustainable Ecosystems, Canberra.

David Salt : Consultant science communicator.

Brendan George: Industry Leader for Private Forestry and Bioenergy, Department of Primary Industries, New South Wales. Richard Harper: Principal Research Scientist, Forest Products Commission, Western Australia. Ted Lefroy: Professor and Director, Center for Environment, University of Tasmania. Nico Marcar: Principal Research Scientist, CSIRO Forest Biosciences. John McGrath: Manager, Science and Resources, Forest Products Commission, Western Australia. Melissa Millar: Research Scientist, Department of Environment and Conservation, Western Australia.

Keith Smettem : Professor and Research Director, Centre for Ecohydrology. Head, School of Environmental Systems Engineering, University of Western Australia. Lynley Stone: Research Scientist, Department of Environment and Conservation, Western Australia. David Thompson: Resource Economist and Principal, Centre for Agricultural and Regional Economics Pty Ltd (CARE), Armidale, New South Wales. Albert van Dijk: Research Scientist and Leader, Terrestrial Remote Sensing and Ecohydrology Research Team, CSIRO Land & Water, Canberra.

Acknowledgements

This book and DVD are products of the Cooperative Research Centre for Plant-based Management for Dryland Salinity (CRC-Salinity). The majority of the chapter authors were members of this CRC, and the CRC provided financial support for development of the book and production of the DVD. The editors are very grateful to the CRC’s executive, led by Kevin Goss (CEO), for this support. The CRC-Salinity completed its mission in 2008 and re-emerged as the Future Farm Industries Cooperative Research Centre (FFI-CRC). The knowledge presented in this book lies fully within the vision of FFI-CRC to develop profitable perennial land management systems for Australian landscapes. Bringing this book together would not have been possible without the energy and patience of many people. In particular we would like to thank the authors for their patience in waiting to see the book come to press. Some of them have had to wait four years, and make the appropriate revisions, from their first submission to final chapters published here. We thank Chris Twomey (CRC-Salinity) for his direction and camera work on the Tree Measurement video on the DVD. We are also grateful to the following for reviewing chapters in the book:

Craig Barton, Mark Burgman, Leon Bren, Helen Cleugh, Tim Ellis, John Field, John Kellas, Vivian Turner, Mark McCrone, John McGrath, Richard Moore, David Pannell and David Thompson. The editors would also like to thank the staff of CSIRO Publishing, in particular Tracey Millen and Ted Hamilton, for their patient guidance through to the smell of printer’s ink. We also appreciate that our respective employers, University of Adelaide, University of Melbourne and NSW Department of Primary Industries, allow us the opportunity to engage in ventures such as this. We leave the final and most grateful acknowledgement to Daryll Richardson from Agave Education. Daryll, as Education Officer for both CRC-Salinity and FFI-CRC, contributed to this book from the very first meeting in Wagga Wagga to the final product. He was responsible for copyright documentation, communications, data storage and especially DVD design and production. Apart from the intellectual contribution, the time he invested in this project was far in excess of his contractual obligations with the CRC. Yet he gave this time without hesitation and always with great enthusiasm. This book would never have seen the light of day without his input.

1 Agroforestry as integrated natural resource management Ian Nuberg, Rowan Reid and Brendan George

Integrated NRM: a time of great change for the better ‘It’s a great time to be involved in the management of agricultural lands and forests in Australia.’ If we wrote that sentence 100 years ago, we would have probably substituted the word ‘management’ with ‘opening up’ and ‘exploitation’. We might have added that this was essential to secure Australia’s place as one of the leading economies and nations of the modern world. While the focus of the task is very different, the scale and urgency of the many issues facing land management in Australia today has not diminished. As we continue to develop the economic productivity of our land we are confronted with the question of how to sustain this development. Agriculture and forestry both impact, and are impacted upon by, soil and water resources, native and feral plants and animals, and by atmosphere and climate. The people who manage the land are responsive to community perceptions of appropriate land management, either by legislative coercion or by gradual changes in their own environmental awareness. The agriculture and forestry sectors share much in their response to the future, and for those involved there is a great opportunity to contribute to the development of truly sustainable and economically viable rural landscapes that are uniquely Australian. The challenge of the early 20th century was to remove the forests and woodlands to make room

for agriculture; the challenge for the future is understanding the interactions between forests and farming and designing new agricultural landscapes that integrate them for conservation and profit. Agroforestry, a marriage of forest and agricultural science, is a pivotal discipline in the practical implementation of this change. This chapter lays the foundation for appreciating the urgency and nature of this change. We begin with a robust and pragmatic definition of agroforestry. Then follows an explanation of how agroforestry emerged in the efforts to secure sustainable agriculture and forestry in Australia. We mount our argument for agroforestry as a key strategy for on-ground action that integrates many natural resource management issues. Moving from theory to practice, we describe the design and planning processes by which farmers integrate their individual vision and resources with real and meaningful changes on the ground. We then introduce the rest of this book by summarising and linking each of its chapters.

What is agroforestry? The terms agroforestry and farm forestry are used interchangeably in Australia with respect to the establishment and/or management of trees on farms for productive purposes. We have chosen the term agroforestry for this book because it reduces the emphasis on timber production, thus acknowledging the equally important role of non-timber 1

2


Increasing scale of planting

Land protection plantations Broad acre planting

Industrial plantations Integrated whole farm agroforesry

Farm forestry plantations

Water table control

Break of slope plantings

Biodiversity plantings

Windbreaks and shelterbelts

Industry plantations on leased land

Industry plantations on owned land

Joint venture plantations Small scale farm Woodlots

Streamside plantings

Increasing emphasis on timber production Figure 1.1: Farm forestry distinguished from industrial forestry and land protection plantings on the basis of scale and emphasis on timber production (Prosser 1995).

products. It also particularly recognises the use of trees and shrubs on farms to support agricultural production, protect soil and water resources, enhance biodiversity, sequester carbon, and improve landscape values. At its simplest, agroforestry, in our view, is a useful all-encompassing term for the deliberate management of trees and shrubs on farms. Nonetheless, specific or comprehensive definitions are important. Shared definitions improve understanding, particularly with respect to new land management practices. Definitions are also important for policy-makers and funding bodies. For example, the increasing government support for farm forestry in Australia during the 1990s generated a great deal of debate over whether farm forestry was different from forestry on farm land. Alexandra and Hall (1998) highlighted the importance of a clear distinction because ‘the lumping of all forestry together tends to blur the issues which are important to farm forestry’. Detailed definitions of plantation types, they argued, ‘are required, not for pedantic reasons but because, by accurately recognising the differences, polices and programs can be targeted accurately’. The National Policy Director for Australian Forest Growers, Alan Cummine (1999), acknowledged that political forces were instrumental in driving sector and government support for particular definitions during the 1990s. He suggested

that the industrial sector, while initially keen to discredit farm forestry, were nonetheless seeking to promote the concept of a seamless continuum between industrial forestry and farm forestry when funding for the National Farm Forestry Program was increased in 1995. This is evident in the model for defining forestry-related land management practices produced by Prosser (1995), representing the National Association of Forest Industries (Figure 1.1). His model was later adopted by many agencies including government departments responsible for administering farm forestry research and development (Donaldson and Gorrie 1996). Within a few years, social research commissioned by government in response to community and environmental concerns over the rapid expansion of corporate or industrial plantations on formerly family-owned farms rekindled debate on terminology and definitions. Pearson et al. (2000) argued that ‘language is important and acceptance of farm forestry is made more difficult when it is confused with social, stakeholder and environmental issues which relate to plantation or industrial forestry’. Schirmer (2000) agreed, confirming that among those in the rural communities where industrial forestry is seen as a threat, farm forestry or the ‘development of plantations on agricultural land owned by farmers’ is seen as very different from

1 – Agroforestry as integrated natural resource management

Table 1.1: Ecological interactions in agroforestry systems, showing positive (+), negative (-) and neutral (0) impacts on the tree and other components in the system (i.e. crop, animal, pest) Interaction

Between (tree, non-tree)

Examples

Competition

(-,-)

Competition between plants for light, water, nutrients and space.

(0 , -)

Allelopathic interference

Predation

( - ,+ )

Predation of insects pests by birds Herbivory by pests and livestock Parasitism of productive and pest species

Mutualism

( +, + )

Mycorrhizal associations Nitrogen-fixing associations

Commensalism

( 0 ,+ )

The effect of shelter provided by trees on the growth and development of plants and animals Cycling of nutrients of one species via surface litter, organic matter and soil fauna to be available to another species

Source: Modified from Anderson and Sinclair (1993)

industrial plantation forestry even if it involves the same species grown in a similar manner. The discussion of plantations on agricultural land continued through the 1990s, but there was also a growing interest in non-timber species (for example for fodder, shelter, groundwater management and non-timber products) recognised as a form of agroforestry without dispute (RIRDC 1992; Bird et al. 1994; JVAP 2002). A definition for every purpose

Internationally, the definition adopted as the basis for an international scientific journal (Agroforestry Systems Journal) and the International Council for Research into Agroforestry (now the World Agroforestry Centre) in the early 1980s attempted to provide a scientific basis for agroforestry research and acknowledge the wide diversity of existing land use practices that might be included: Agroforestry is a collective name for land use systems and technologies where woody perennials are deliberately used on the same land unit as agricultural crops and/or animals, either in some form of spatial arrangement or temporal sequence. In agroforestry systems there are both economical and ecological interactions between the different components (Lundgren 1982). This definition is cumbersome but does contain some important elements, especially in defining how agroforestry science might be distinguished from other formal disciplines. First, the term

‘woody perennials’ includes not only trees but shrubs, palms and bamboos;. Some of these are important elements of integrated farming systems. Second, the definition emphasises the integration of the woody perennials with agricultural species (crops, pastures, animals) and acknowledges that this may occur at a number of scales, from paddock to farm or even landscape scale. It may also occur over time. Phase farming systems are an example of temporal integration where productive trees may be used to draw down a watertable, then clearfelled, and thereby make the ground suitable for an agricultural phase (see Chapter 3). A third and most critical point is that in agroforestry systems there is an interaction – physical, environmental or economic – between the agricultural and tree or shrub components. This interaction can be positive or negative. The science of agroforestry focuses on understanding these interactions so that land managers can better design and manage systems that minimise the negative and maximise the positive interactions in a way that best satisfies those involved. The ecological interactions between these components (Table 1.1) underpin the economic interactions. Even without any ecological interactions the inclusion of trees into a farming system might complement agricultural production by diversifying income or better utilising farm labour or equipment. These economic interactions can be just as significant as the environmental or agricultural benefits that drive most agroforestry projects.

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One of the difficulties of assessing the net result of the many interactions is that some of the benefits of growing trees may not accrue for many years. Similarly, if the trees are being grown to minimise the impact of a possible environmental or agricultural risk, such as soil erosion, drought, bad weather or failing markets, it may be difficult to quantify the impact of the trees. There may be no way of telling when the trees will be required. Agroforestry design and management is very much more complex than simply selecting a planting configuration or mixture of trees and farming that appears to be more productive or sustainable than farming alone. Although the International Council for Research into Agroforestry definition provided a useful basis for physical research for the development of a new scientific discipline, it was not appropriate for those outside science. With an eye on the political realities of government funding and public promotion, there has been a tendency to include the anticipated benefits of agroforestry and farm forestry in definitions. For example, when ICRAF was renamed as the World Agroforestry Centre in 2001 it redefined agroforestry as: A dynamic, ecologically based, natural resource management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic and environmental benefits for land users at all levels (FAO 2008). The Australian Joint Venture Agroforestry Program (JVAP) provides and manages funds for research on agroforestry and farm forestry, and facilitates delivery of that R&D to industries, communities and government. Its definition of agroforestry and farm forestry presumes the intention of the land manager: ‘Agroforestry or farm forestry is the incorporation of trees into farming systems, for commercial and natural resource management benefits’ (JVAP 2006). Reid and Stephen (2001) argue that it is not appropriate to define farm forestry as a predefined set of land use practices or to distinguish it from other forms of revegetation on the basis of scale or intention. Nor is it proper to embellish the definitions with attractive outcomes that suggest agroforestry or farm forestry is more profitable or

sustainable than alternative land uses. They suggest that what clearly distinguishes a farm forest or an agroforest from a corporate, industrial or government forest is ownership. Not just ownership of the land or the trees, but ownership of the decision to do it and how it is done. Reid and Stephen argue that farm forestry and agroforestry are terms that relate not to the outcome but to the process by which these forests are established and managed, and that it is this process of farmer decision-making that should guide the research and development of agroforestry. They offer the following definition: Farm forestry (or agroforestry) is the commitment of resources by farmers, alone or in partnerships, towards the establishment or management of forests on their land. Just as there is little need to define agriculture or forestry, other than what is done by those who see themselves as agriculturalists or foresters, there is no need to define agroforestry on the basis of what it looks like. As Reid and Stephen (2001, p. 7) say: Farmers may establish and manage their forests for any mix of the benefits they might provide. They may place an emphasis on a single outcome, such as timber production or biodiversity, or they may seek to balance a range of benefits in a multipurpose planting. Their priorities may vary over the farm and change over time. A forest initially established or managed for wildlife or land protection might later be harvested for timber or valued for its beauty (amenity value). Forests on farms may increase agricultural production or simply displace it. They might be sustainable, even improve economic, social and environmental capital, or they may deplete these assets. The farmer, or their partners, may profit from farm forestry or come to regret their involvement. Making a commitment to forestry is not necessarily a good decision – it is simply a decision. The goal of this book is that: knowledge and experience can improve decision-making, thus helping all land managers, and those working with them, to design and maintain landscapes that better meet their economic, environmental and social aspirations. If that involves the establishment and management of trees on farms, then it might be called agroforestry.


For those involved in the plant and animal sciences of agroforestry, the World Agroforestry definition which emphasises the interaction between the tree and agricultural components of the system provides a clear basis for research. For those more interested in policy, definitions of agroforestry or farm forestry that focus on the ownership of the land may be more appropriate. Our interest, for this book, is the role that trees and forests can play in supporting and sustaining farming systems in Australia. It may not be important whether the trees are integrated with agricultural crops or stock. Neither may it be critical who owns the trees or the purpose for which they are being grown. For us, agroforestry is about the deliberate establishment and/or management of trees in the agricultural landscape of Australia.

Agroforestry in Australia How and where agroforestry develops in Australia will reflect the interests and motivations of the farmers involved, and the preparedness of various interest groups to reward those who are able to provide the products and services the community value. Some will be consumers of particular products, such as timber or water. They will look to agroforestry as a means of improving the supply and quality of the particular products they are seeking at a price that they are willing to pay. Others will be more interested in the potential for agroforestry to support farmers by improving agricultural productivity, ameliorating land degradation and improving farming incomes or viability through diversification of income. A third group of stakeholders is concerned about broader environmental and social issues such as biodiversity, climate change, social stability and regional development. All the expectations and views of the respective groups are valid, but the link between the groups can be tenuous. Generally, large-scale change is underpinned by tangible markets that provide an income from agroforestry activities. Much has been made of the great potential for agroforestry to become a valuable contributor to the economic and environmental well-being of rural Australia (Reid and Wilson 1985; Alexandra and Hall 1998). Agroforestry, in its different forms, represents a significant proportion of Australia’s

native forest estate and if the trend towards increased plantation establishment continues then farm forestry could become an important component of planted forests. In comparison, in North America more than 10 million non-industrial private forest owners collectively manage approximately half the national forest estate and surpass the combined timber production from industrial and government forests (Biles 2001). However, despite a dramatic increase in the number of farmers taking an active interest in agroforestry and the rise in the financial and political support from government, industry and community groups in Australia, it remains difficult to ascertain if this potential will be realised in the short term. In this book we review the potential of agroforestry to produce various wood products. We introduce the issues and opportunities facing those establishing and managing agroforestry systems for non-wood products of commercial value. If these forests can also address some of the opportunities for enhancing and sustaining agricultural production, for example by reducing soil erosion or offering protection for livestock, their attraction for farmers is obvious. Agroforestry can help meet some larger social and environmental objectives, such as catchmentbased biodiversity goals. The key issue for landholders whose actions provide these community benefits is the lack of tangible rewards. The relationship between the on-farm and off-farm impacts of agroforestry is therefore critical. Agroforestry systems designed and managed to enhance the viability of the farm may also provide catchment and community benefits. For example, a well-established and maintained windbreak can lead to reduced soil erosion or an improvement in the livestock survival rates from key operations such as lambing. These outcomes will be at a local or farm level and may directly (e.g. increased lambing) or indirectly (e.g. reduced erosion) assist with the sustainability of the agricultural production systems. They will also reduce the off-farm impact of soil erosion, including dust storms. The most difficult component of valuing the non-wood returns from agroforestry relate to larger sustainability issues and concerns such as biodiversity. There is clear evidence that planting of native trees, especially in mixed species stands, can

5

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dramatically improve the biodiversity habitat of many species (Kavanagh et al. 2005). But how do we establish a system where land managers can do this and gain some economic return? The lack of markets does not detract from the importance or the biophysical need to consider increased mixed species planting to meet these non-wood opportunities. However, the lack of economic certainty limits the capacity of land managers to invest. To increase the appeal of agroforestry we need to understand not only the markets and wood and non-wood values, but also the key social and biophysical drivers. Timber production as a driver for agroforestry development

For large-scale development of any industry it is important that there is a sound economic base and a capacity to operate in domestic and international markets. Due to our large land area and efficiency of production, the majority of Australian agricultural commodities are exported (approximately two-thirds). This varies across the sectors but is expected to increase. Most of the trade now occurs with countries within the Asian sector. In 2005–06 the value of Australian forestry exports was estimated at $2.1 billion, predominantly to countries in the Pacific region including Japan, New Zealand and China. By value, the major export commodities were woodchips ($839 million), paper and paper products ($593 million), panel products ($151 million) and sawn timber ($118 million) (Commonwealth of Australia 2007). In the same period Australia imported approximately $4.0 billion in wood products. The majority of the imports were value-added forest products, predominantly paper, paper manufactures and paperboard ($2.6 billion), miscellaneous forest products, such as furniture ($528 million) and sawn wood ($419 million). The large trade deficit of approximately $1.9 billion acts as an incentive to develop the forest industries that can take low-value products, such as woodchips, and convert them to high-value products, such as paper products. Further, there is a significant opportunity to develop high-value sawn timber. This is of particular interest to agroforestry managers in valuing a longterm product (the sawlog) while managing the system to deliver on agricultural products and environmental services.

The sector needs to yield a competitive return on the investment, however, to be favourably considered. And the investors need to understand some of the risk associated with their investment activities. Agroforestry systems can help address both aspects of financial gain and, with consideration, larger economic gain for the community. Appropriate government policy and initiatives are critical to meeting these targets. Much of the policy developments in Australia over the last 15 years have been driven by the National Forestry Policy statement (NFPS) delivered in 1992 (DAFF 1995). The goals offer a strategic direction for private native forests and plantations. For native forests, the NFPS goal is: Ensure that private native forests are maintained and managed in an ecologically sustainable manner, as part of the permanent native forest estate, as a resource in their own right, and to complement the commercial and nature conservation values of public native forests (p. 4). The NFPS clearly recognises the importance of private native forests. Their capacity to contribute to the total value of native forests wood products has increased significantly since the introduction of the Regional Forest Agreements in the mid 1990s. The NFPS goals for plantations are: Expand Australia’s commercial plantations of softwoods and hardwoods so as to provide an additional, economically viable, reliable and high-quality wood resource for industry. Other goals are to increase plantings to rehabilitate cleared agricultural land, to improve water quality, and to meet other environmental, economic or aesthetic objectives. The scale of plantations has increased in Australia since the end of the 20th century. This activity is underpinned by Plantations for Australia: The 2020 Vision (Plantations 2020 2003). This strategy aims to guide the sustainable expansion of the plantation forest estate, including significant private sector investment. By 2020, the expanded plantation forest estate will provide Australia’s plantation-based processing industries with the capacity to:

s s

operate in the global marketplace; be internationally competitive;


s

be commercially oriented – market-driven and market-focused in all their operations (Plantations 2020 2003, p. 5).

Thus the development of plantations is intended to increase the production of wood and wood products, especially from private investment and generally based on cleared agricultural land. Importantly, the 2020 Vision states that ‘returning trees to the landscape as a profitable crop can also significantly benefit rural and regional communities and the environment’ (Plantations 2020 2003, p. 5). However, this objective leads to conflict in some areas over production-scale forestry replacing cleared agricultural land operations. Carbon markets and other drivers for agroforestry development

Not only is the potential value of wood products changing in a dynamic international market, but so are the expectations of delivering non-wood products and values from government and privately controlled land. This recognition is being followed, albeit slowly, by developing markets such as carbon trading. Nicholas Stern, when reviewing the potential economic impact on climate due to changing greenhouse gas concentrations, described it as the greatest market failure the world has seen (Stern 2007). The potential impacts from climate change on life, human lifestyles and the economy in Australia are highly significant and calling for urgent action (Garnaut 2008). The potential mechanisms for international carbon markets are described in Yamin (2005) and Australia has responded with its Carbon Pollution Reduction Scheme (Australian Government 2008). While agroforestry will have a role in addressing some of the market drivers (e.g. sequestration of carbon within an emissions trading scheme), the opportunity and influence on agroforestry activity should be viewed within the context of other nonwood products and issues. The relative importance of the drivers may change (for example, the introduction of the Carbon Pollution Reduction Scheme in Australia will lead to the development of a price for carbon and forestry activities benefit from early participation). But the development of robust agroforestry activity needs to account for multiple out-

comes for large-scale implementation. Some of these considerations, from an economic perspective, are outlined by Thompson and George in Chapter 18. Not only will agroforestry be able to meet opportunities through mitigation of climate change, but there will need to be further consideration of adaptation for the long-term survival and productive capacity from the systems. Climate change will potentially lead to altered conditions for species as temperatures increase, rainfall patterns shift and the general climatic patterns become more variable. These issues are beyond the scope of our work but should be considered in the planning and management of future agroforestry systems, especially where species may already be at the limit of their natural ecophysiological range. Other social or environmental drivers of land use and management change include the development of peri-urban activities, where small-scale farms are being managed by people with off-farm income. There is significant opportunity for agroforestry systems to meet some of these non-wood values, but planning is required. There will always be an opportunity to balance the wood and nonwood values from agroforestry systems, but for large-scale uptake there must be markets that will allow land managers to remain economically viable. The value of wood products is important to regional areas and Australia; we now consider some of the scale of the forestry output. Production value of agriculture and forestry in Australia

Australia is the most urbanised country in the world, with approximately 84% of its (now) 21 million people living in the major cities around the coast (Hamnett and Freestone 1999). Though now predominantly urbanised, Australia has historically relied upon significant primary production, especially for international markets. Following the fundamentals of economic development, primary production has retreated from that dominance as other sectors of the economy advance. Today, primary production directly contributes only about 3% to the Gross Domestic Product. Nevertheless, in 2005–06 the value of Australian agriculture was estimated at $38.5 billion while the gross value of logs removed from forests (value at the mill door) estimated at $1.7 billion (DAFF 2007). An important

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Table 1.2: Land use in Australia Land use Plantations

Area (Mha) 1.8

% of total land area 0.2

Agriculture Agricultural and horticultural crops

26.7

3.5

442.4

57.5

Total

469.1

61.0

Native forests and woodlands

162.7

21.2

11.4

1.5

Grazing

Public native forest where timber production is permitted Forests in nature conservation reserves

21.5

2.8

129.8

16.9

Total

162.7

21.2

Total land area

766.0

100.0

Other categories

Source: Parsons et al. (2007)

aspect of the forest industry is the value-adding along the supply chain. Wood and paper products, for example, support the manufacturing and construction sectors of the economy. There are about 155 000 farm enterprises in Australia (ABS 2008) and forestry is the secondlargest manufacturing industry, with 83 000 jobs (DAFF 2007). Agriculture and forestry are critical for employment and local economies for the estimated 16% of the population living in rural areas. Apart from the economic links between country and city inhabitants, agriculture and forestry together are highly significant in their impact on the ecological services provided by natural resources that all Australians rely on. Schirmer et al. (2005) estimated that, in the south-west slopes of New South Wales, for each dollar invested in plantation forestry approximately $1.63–1.81 was generated within the local economy. Of this, some $0.31–0.49 was spent in local wages. This has significant implications in areas where farm returns have declined over long periods and the capacity to increase local employment exists, especially through processing of wood products. The size of the Australian agricultural sector is dependent on market conditions (including the Australian dollar and commodity prices) and on local environmental constraints such as drought. For example, in the summer of 2007 around 60% of broadacre farmers indicated their properties

were drought-affected. This is the same proportion that was drought-affected in 2002 (ABARE 2007). The effect of drought is expected to worsen with climate change impacts – the capacity to adapt and mitigate the problem is critical. Land use and agroforestry

Agricultural and public production forestry utilise significant areas in Australia to support the production of primary industries goods and services to the economy (Table 1.2). There are approximately 163 Mha of forests in Australia covering about 21% of the land area and comprising just over 4% of the world’s forests. About 13% of the forest estate is formally protected in nature conservation reserves (BRS 2007a). The majority (approximately 70%) of Australia’s native forests are controlled through private management, via leasehold or outright ownership. Thus private forestry has a significant role not only in meeting wood and wood products demand but also in managing forests to achieve better environmental outcomes through maintaining or enhancing biodiversity, water quality and carbon sequestration. However, the lack of transparent markets in these non-wood values limits some of the opportunities for the land managers. These managers carry significant costs, reducing production capacity to meet environmental policy (Productivity Commission 2004).


Table 1.3: Estimated areas of plantations located on farms across Australia, 2005–06

State

Total plantation area

WA

377 598

Unknown plantation

Hardwood

Softwood

270 813

104 480

2305

Estimated area ‘farm forestry’

% forestry on farms

65 702

17.4

NSW

331 623

55 196

273 606

2821

27 856

8.4

Vic.

384 599

164 724

218 412

1463

25 384

6.6

Tas.

227 200

155 500

71 600

100

21 357

9.4

Qld

225 637

37 496

186 033

2108

4738

2.1

NT

16 329

14 090

2239

0

425

2.6

SA

8.6

166 962

42 341

124 164

457

14 359

ACT

9500

0

9500

0

0

Total

1 739 448

807 437

990 034

9254

159 820

0 9.2

Source: BRS (2007a)

The 2007 National Plantation Inventory Update (BRS 2007b) indicates that in Australia there are about 1.82 Mha of plantations. Some 807 500 ha are hardwoods, almost 1 M ha are softwoods and 9254 ha are unidentified (Table 1.3). Between 1997 and 2007 most of the increase in plantations was for hardwood investment. For example, in 2006 there were 67 200 ha hardwood established compared to 11 100 ha of softwoods. Most of the plantation development is carried out by companies linked with managed investment schemes. The National Farm Forestry Inventory (NFFI) indicates that over the last thirty years there has been a significant increase in the area of small grower plantations (Stephens 2001). Table 1.3 shows that only about 9% of plantations are located on farms. While the penetration of forestry into the agricultural sector is good for correcting the trade imbalance for wood products and rehabilitation of degraded agricultural land, a balance must be found between changing land uses. Only 0.2% of the land resource is under plantation forestry, but there is a common perception of conflict with ‘traditional’ agricultural land use. As agricultural landscapes change with tree plantations, there is considerable concern about the social, economic and environmental impacts of large-scale monoculture eucalypt and pine plantations (Spencer et al. 1989; Schirmer 2000; Hopton et al. 2001; Schirmer et al. 2005). In this context, the way that agroforestry is implemented becomes important in balancing the needs of production and scale with local

community perceptions and aesthetic values. This leads us to consider the people in these communities who will implement these changes to land use and production. Human capital and agroforestry

As a group, farmers are getting older. The average age of farmers in 2006 was estimated to be 52, and it is increasing (ABS 2008). The next generation has less interest in managing the family farm. There are significant demographic changes in rural Australia, which mean there will be new attitudes to planting trees on farms. Many of the younger farmers who take control of existing family farms (and sometimes significant debt) need to maintain profit and cash flow. They will not be interested in broad-scale agroforestry unless it is a commercially competitive option. As detailed in Chapter 17, they will assess an agroforestry investment with a high discount rate. Other landholders, especially those near large regional towns or cities, are more inclined to focus on lifestyle over production or look at lower input systems (Barr and Wilkinson 2005). The ‘treechangers’ who move from urban to rural areas for lifestyle reasons may view agroforestry with a low discount rate, i.e. environmental and aesthetic values dominate over future financial return. Age, attitude and financial means influence the individual likelihood and capacity to adopt agroforestry. Efforts to enhance the adoption of agroforestry to increase natural resource management outcomes will need to consider the heterogeneity of the audience and their

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goals. The extension strategies used to support agroforestry are detailed in Chapter 19. We need to accept that provision of information alone will not necessarily increase the adoption of agroforestry (Pannell et al. 2006). The mechanisms of government policy for enhancing adoption are discussed in Chapter 18.

Agroforestry for integrated NRM Agriculture and forestry are conventionally considered as separate sectors of the natural resource economy with traditionally different physical resource bases, geographic distribution and ownership and investment structures. Agroforestry straddles the two sectors, as the traditional boundaries are being blurred. The case has been made that drivers in the forestry sector will ensure that farm-grown trees will be an important component of that sector; the essential driver in the forestry sector is economic opportunity in a world market increasingly requiring more wood products. Earlier sections outlined changes in the resource base and social structure in the agricultural sector that underpin the increasing role of trees in agriculture. We could argue that the essential driver for agroforestry in the agriculture sector is the need for sustainable and profitable management of the natural resource base. While agriculture is a still a significant contributor to Australia’s export income it has declined in relative importance to other sectors in the national economy. This decline has been shadowed by changes in the demography of rural populations and their political influence. Agricultural research has evolved from a focus on increasing productivity to considering the sustainability of agricultural systems, particularly in relationship with larger landscape processes. All this change has occurred across one or two generations of farmers – farmers who will be caring for the land in 2020 will have a very different environment from their predecessors. In aggregate, they will be wealthier and manage larger, more technologically and commercially sophisticated operations which will be more closely and efficiently linked into global commodity supply chains (Barr 2003). These farmers will be widely recognised for their role in managing the landscape to sustain its ecological services, particularly water, carbon and

biodiversity. Agricultural systems will have to be more closely adapted to Australia’s fragile soils, highly variable seasons and increasingly changing climate. Some agricultural systems may, to some degree, mimic natural ecosystems in structure and/ or function (see Chapter 2). We may see a proliferation of new income streams as farmers are paid for non-wood values such as the carbon they store, the biodiversity they maintain and the extent to which they manage water, increase catchment water yield and reduce catchment salinity (Williams and Saunders 2002). Innovative thinking is leading discussion on the value of forest management for environmental outcomes and the concepts of stewardship payments to land managers (Southern Cross Group 2006). Already we are seeing concerted efforts to establish new tree-based industries for pulp and energy production in agricultural areas (Chapter 16). The role of non-horticultural trees in rural landscapes has traditionally been secondary to the field crops and pastures that provide the marketable commodities. Trees and shrubs have had a role in protecting crops and soils (Chapter 5) and livestock (Chapter 13), and in providing fodder (Chapter 14) and landscape amenity (Chapter 8). However, over the last two decades there is general acknowledgement of the importance of farm trees in maintaining the hydrological balance (Chapters 3 and 4) and biodiversity of rural landscapes (Chapter 6). Direct income from farm trees is now a possibility for many landholders (Chapters 10–12), not just the enthusiastic few. There has also been growing awareness of the multi-functionality of rural landscapes beyond providing just grain and meat. Agroforestry is finding a niche in ‘amenity farming landscapes’. Generally, this is the classic rolling green hills where urban escapees retire or seek a tree-change in lifestyle. Similarly, peri-urban landscapes offer an opportunity for small-scale tree planting. The price of land in these landscapes is usually beyond the means of viable farm business investment and the main income of landholders is from off-farm sources. Agricultural income may be niche horticulture or small-scale grazing of beef or sheep, not without attendant problems. The beef industry is increasingly geared to paddock-toplate quality control procedures beyond the means of small producers. Running sheep in more densely


populated landscapes with non-farm dogs also has problems. Finally, weed and pest management can be a significant problem for time-poor or indifferent landholders (Barr 2003). In this context, smallscale tree planting for a long-term investment, perhaps to be realised as part of a local farm-forestry cooperative, offers an aesthetically and ethically satisfying alternative land use. The Decade of Landcare (officially 1990–2000) was largely responsible for the huge expansion of farm tree planting in areas of broadacre agriculture and grazing (Walker 2000). Plantings were mostly for soil conservation, stock shelter and fodder, groundwater control and wildlife habitat. They were the sign of an awakening and cultural change, especially among younger farmers. Landcare relied on committed individual and community efforts, from people who could see beyond the bottom line of annual gross margins. It remains to be seen how firmly embedded the tree-planting ethos is as farm sizes increase, their business structures become more strongly coupled to global markets and the rural population continues to decline. For agroforestry to flourish in this context we will need new tree-based industries for low-rainfall areas. These are likely to be multi-product industries (e.g. fibre and energy) that are supported by incentives from other natural resource sectors, such as energy and water supply. Woody–perennial fodder systems may cover even larger areas of this landscape as they will be more easily integrated into existing agricultural systems. The search for these, as well as herbaceous–perennial farming systems, is central to the activity of the Cooperative Research Centre for Future Farm Industries (2008–2014). Natural resource management is planned and implemented in Australia through 56 regional bodies or catchment management organisations with various structure and functional arrangements (Pannell et al. 2007). The funding of resources has been devolved to the local (catchment) level. This allows for better identification and recognition of, and relationship to, specific issues. For agroforestry opportunities and issues, placing responsibility and action at the local level delivers improved understanding and better onground outcomes. NRM bodies share the task of integrated management of a broad range of natural resource assets,

one of which is perennial vegetation. Agroforestry is a land use option that integrates income generation for landholders with, most notably, ground and surface water management, soil conservation and biodiversity management. It can also be a consideration in the regional management of pests, weeds and fire. Planning and management around these issues relies on a good understanding of the nature and options of agroforestry and a recognition of the dual role of production (e.g. wood) and sustainability from the managed system. Whoever is considering agroforestry, whether at the level of an individual property or as an option that may that may be promoted across a region, will need an understanding of the design and planning processes in agroforestry (discussed below). Identifying the key objectives, and applying suitable species and styles to attain those objectives, is critical in developing a robust agroforestry system capable of delivering a successful outcome.

Design and planning processes in agroforestry There are essentially two approaches to agroforestry design: the ‘best bet’ and the ‘diagnosis and design’ approaches. ‘Best bet’ approach

The ‘best bet’ approach is where a landholder simply copies the species, establishment techniques, planting arrangement and management used or advocated by others. The design and practices being adopted may have been proven effective by industry, researchers or other landholders. This approach can be effective but overlooks the potential, and the possibly the need, to adapt the design and management of agroforestry systems to better reflect the objectives, resources, attitudes and opportunities facing each particular landholder. If agroforestry design were as simple as selecting off-the-shelf packages or recipes there would be little need for a book like this. All that would be needed is a collection of best bets or design options developed by researchers and practitioners for every region. Landholders would simply pick the one that seems to suit. They might prefer options developed by people like them or those used in their region on similar soil types, or they could just

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choose options that appear affordable and appeal to their sense of aesthetics. It would be a bit like going to a used-car yard – you look at all the models on offer and pick the one that seems the best, fits your budget and matches your self-image. It is easy to assume that every tree-grower faces the same constraints (e.g. limited land, time or money) and measures success in the same way (e.g. discounted returns per hectare). In our experience this is not always the case (see Chapter 17). As every farm family and every farm is different, it follows that the most appropriate agroforestry design for each will be unique. People have different attitudes to investment risk, measure profit in different ways and have diverse views about the likely future value of different products and services. And these attitudes may change over time. The situation is even more complex where landholders are seeking a mix of commercial and non-commercial benefits from their trees. Diagnosis and design

As Andrew Campbell wrote when he was the National Landcare Coordinator, ‘the complexities inherent in sustainability and the primacy of farmers in making land management decisions mean that a recipe approach to land management recommendations won’t work’ (Campbell 1994, p. 200). Fortunately the alternative is not only practical, it is less risky, more affordable and likely to be much more rewarding. It involves a diagnosis of the concerns, opportunities and aspirations of the landholders, a review of the role that trees and forests might play, the design of possible agroforestry systems and evaluation on the basis of the rewards, costs and risks associated with each. The importance of this book lies in the fact that a landholder with clear objectives and some basic knowledge can design and manage an agroforestry system that best matches their own circumstances, acknowledges their particular exposure to risk and maximises the suite of rewards that are most important to them. The diagnosis and design approach provides a guide to developing unique agroforestry designs for each situation. The key difference is the starting-point: rather than having a particular species, layout or management plan in mind, it starts with a review of the landholder’s particular circumstances and

the role that agroforestry might play in addressing them. Then, based on the landholder’s individual constraints and goals, possible designs are developed and tested. Their advantages and disadvantages are highlighted in a way that informs landholder decision-making. An important advantage of this approach is that it acknowledges that forests planted for a particular purpose can be designed and managed to deliver a range of benefits over time. For example, the need for land degradation control or stock shade and shelter may define where trees must be established on a particular property and the role they must play to be effective in the short term. Then, if the landholder has an interest in producing timber, they may consider how to adapt their design to incorporate commercial species and forest management options that keep alive the possibility of harvesting a commercial product in the future (see Figure 1.2). Much of this book is dedicated to providing an understanding of the processes underlying common land management problems and opportunities facing landholders across Australia. Chapters 2–8 describe the underlying processes and role of growing and managing trees on farms in meeting these needs. Chapters 9–16 go further by reviewing the growth and development of trees and exploring the management options available to landholders interested in producing commercial tree products and services. It is this type of knowledge, coupled with the aspirations and experience of landholders themselves, which is required to design and manage successful appropriate and adaptable agroforestry systems. Planning tools for agroforestry

There are a number of planning tools and techniques that are can be used during the diagnosis and design process. Some are systematic or stepwise techniques for planning the most appropriate location of trees on a farm; others are conceptual tools for exploring the structural design and management options that may be feasible. Data overlay method Many landholders are now familiar with using an aerial photograph and a series of plastic overlays as a planning tool to determine the location of fences,


BEFORE

Grazing causes erosion, reduces vegetation cover and pollutes water courses

Run-off from paddocks carries nutrients, chemicals and soil

Waterlogging causes slumping of banks

Stream flow undermines banks and erodes unprotected soils

Opportunity:!Trees and vegetation for erosion control !ence out stock!Tall trees for shelter

AFTER Tall trees pruned for timber production provide shelter for paddocks, wildlife habitat and aesthetic values Tree roots dry out banks and reduce slumping

Grasses trap sediments from overland flow Shrubs and ground vegetation protect the banks from erosion and provide wildlife habitat and low shelter

Water plants protect the floor of the stream

Figure 1.2: Appropriate agroforestry design for multiple outcomes begins with an understanding of the problems and opportunities. This example of a multi-purpose riparian buffer seeks to achieve a range of commercial and non-commercial outcomes while maintaining management flexibility (Abel et al. 1997).

roads, stock laneways and revegetation projects. Information not shown on the aerial photograph, such as soil types, contour lines, vegetation types, threats, fence lines and recent improvements, can be added to a base overlay sheet, thereby providing more information when exploring possible fencing and planting locations. The increasing availability of physical and production data and software means that the overlay method can be extended to incorporate annual crop-yield data, real-time soil moisture measurements and other information that can improve farm management decision-making. The value for agroforestry design lies in highlighting opportunities for fitting tree-growing into the farming landscape, complementing other farming activities and improving economic and environmental landscape function.

Analogue systems (mimicking natural systems) The term ‘analogue forestry’ was first used to describe an approach to system design and evaluation that promotes options which closely mimic the structure and function of natural forests (Senanayake and Jack 1998). The underlying assumption is that we can learn a lot about the productive and ecological potential of a site by looking at the structure and function of the native vegetation. An example is the use of nitrogen-fixing tree species in eucalypt plantations that mimic the natural species mix common in native eucalypt forests found in the area. In some cases the emphasis might be on the aesthetics rather than functionality. For example, the aim might be to recreate the open woodland or parkland appearance thought to have been induced by Aboriginal fire-stick farming, but using sheep, improved pastures and wide-spaced

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Good agroforestry design allows for change Designing a new tree planting project is a little like designing a house to suit a particular family – a review of the family’s needs and aspirations now and in the future is essential. However, the process of managing a forest is never completed. Forests are dynamic and landholders must continually adapt their management in response to changing circumstances. Good agroforestry design involves exploring opportunities and planning for uncertainty so that the decisions made now provide the greatest prospect of success in the future. Figure 1.3: A good property plan allows farm foresters to integrate their tree plantings with agricultural activities for optimum performance (photo by R. Reid).

commercial timber trees instead of kangaroos, native pasture and old twisted red gums. Flow diagrams Many natural and production systems that occur on farms can be broken down into a number of distinct steps or stages and presented in a flow diagram. Displaying the annual production cycle associated with the production of farm stock or crops as a flow diagram can highlight opportunities for trees to enhance productivity or reduce risk. The stages involved in establishing, managing and harvesting tree products can also be presented in a flow diagram. This may be useful in highlighting when and how much labour is required for particular management activities, planning harvesting operations or identifying when the landholder needs to begin learning new skills or negotiating with buyers. Cause and effect relationships In this book we review the theoretical and scientific knowledge of the processes underlying common natural resource management problems on Australian farms. When coupled with landholders’ experience, observations and on-farm research, this information can help build understanding of the processes that link a particular action (e.g. tree planting) with an outcome (e.g lowering watertables or reducing wind speeds). However, tree planting can result in unintended consequences. Good design requires broad-based thinking about the possible impacts of a proposed agroforestry system on all aspects of the farming system, before the trees are planted.

Summary and invitation to this book There are many good texts that lay the foundations for an understanding of natural resource management (Aplin 1998; Yencken and Wilkinson 2000; Dovers and Wild River 2003). There are some fine books describing and prescribing the practice of agroforestry in Australia (Abel et al. 1997; Reid and Stephen 2001). Agroforestry for Natural Resource Management presents a multi-disciplinary perspective of how agroforestry is being used as a strategy for sustainable management of Australia’s natural resources. The authors come from a wide range of disciplines – agronomy, forestry, community and molecular ecologies, agricultural economics, soil science, hydrology, landscape architecture and rural sociology. They are writing for students and professional practitioners of natural resource management as defined earlier in this chapter. The scope of agroforestry in this book is the planting of trees and shrubs on rural property largely, but not exclusively, with an intention to manage dryland salinity. This will include plantings in high- (>700 mm) and low-rainfall zones in southern Australia. The trees may be grown for timber and other commercial outcomes or for environmental outcomes only. The book is divided into three sections, each with a different focus. Part I: Environmental function of trees in the landscape

This section examines the main environmental functions of trees in the landscape. It opens with Ted Lefroy’s discussion, ‘Agroforestry for functional mimicry of natural ecosystems’.


The powerful concept that our agricultural systems should mimic the structure and function of the natural ecosystem has encouraged scientists to look for productive land management systems that are in tune with the landscape, that do not deplete resources and degrade the land. As most natural land ecosystems are dominated by woody perennial species, agroforestry is an obvious way to re-invent our agricultural landscape. Lefroy provides challenging answers to important questions. Is it necessary to mimic structure to achieve functional goals? Does perenniality inevitably imply a trade-off in productivity? Can competition be managed in synthetic polycultures of trees and crops? How can complex farming systems be successfully managed to ensure a return to the farmer? An important function of agroforestry in some landscapes is to control groundwater and thereby mitigate dryland salinity. Land managers have to integrate a lot of difficult science to develop and implement revegetation strategies. It is equally important to know when revegetation is not the answer, when other strategies, such as drains, are more appropriate. In Chapter 3, ‘Using trees to manage local and regional water balances’, Keith Smettem and Richard Harper provide the fundamentals for understanding the hydrological and physiological processes behind the use of perennial vegetation for mitigating salinity. They offer advice for the strategic placement and configuration of tree planting in the landscape. That chapter is complemented by the next, ‘Agroforestry for the management of water, salt and agricultural diffuse source pollutants’, by Tim Ellis and Albert van Dijk. Where Chapter 3 focuses on groundwater and salinity, Chapter 4 discusses the management of surface water and the pollutants it carries, including sediment and nutrients responsible for eutrophication as well as salt. It describes the hillslope scale processes by which tree plantings may capture water and pollutants and their effects on stream flow and dryland salinity at catchment scales. It provides an insight into the type of experimental and modelling work undertaken to understand these linked stream–groundwater systems. Guidelines on the design and placement of agroforestry plantings are provided specifically for the purposes of surface water and pollutant management. The scale of revegetation required to restore the pre-agriculture hydrological balance is vast. Most

of this landscape is under broadacre crops and pastures. We cannot afford to blanket the landscape with trees at the expense of agriculture. The shelter provided by trees is often touted as a commercial justification for integrating tree-planting with broadacre agriculture. In Chapter 5, ‘Trees protecting dryland crops and soil’, Ian Nuberg and Mike Bennell examine this concept in the light of international and recent Australian research. They show that trees can provide considerable shelter to dryland crops and are particularly important during extreme wind events in dry years. We are fooling ourselves if we believe that windbreaks will elevate net crop yields over large areas of Australia, but they can insure against storm damage to crops and soil as well as enhance livestock production. Protecting and enhancing biodiversity is another compelling reason for promoting agroforestry. Agroforestry plantings cannot substitute for native vegetation communities but they can provide some of the resources necessary for some regional wildlife and native plants. David Salt and David Freudenberger, in ‘Biodiversity and habitat enhancement’ (Chapter 6), help us understand biodiversity and how agroforestry can enhance it in agricultural landscapes. They provide very practical guidelines to follow in planning and managing an agroforestry enterprise for biodiversity. The chapter includes case studies where these principles have been applied. The simple act of planting trees does not necessarily improve biodiversity. Indeed, as Chapter 5 explains, a poorly designed plantation can be an environmental liability. Not only can it have little positive value for local flora and fauna, it can have a serious negative impact. Most people relate this to exotic tree species, such as pines, and consider any Australian native species a more benign alternative. However, in Chapter 7, ‘Environmental risk in agroforestry’, Margaret Byrne, Lynley Stone and Melissa Millar describe the weed and genetic risks associated with the use of native species for largescale revegetation for agroforestry, even within their natural range. This is not a call to limit agroforestry to protect biodiversity, but to employ risk management frameworks in the implementation of revegetation programs. The final chapter on the environmental functions of agroforestry is Chapter 8, ‘Landscape aesthetics and agroforestry’, by Grant Revell. Revell

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uses the word ‘landscape’ in a very specific way. The landscape is the way we sense the environment, filtered through our values and belief systems. Our perceptions about whether a landscape is being well or poorly managed are grounded in our culture. While land managers may think that they are managing biophysical resources for the private and public good, Revell shows how they are also the creative managers of cultural symbols. How do we make environmental features important for biological reasons and economic values important for landscape aesthetic reasons? Revell discusses different paradigms of landscape assessment according to their resource management priorities. The chapter offers design guidelines and case studies that illustrate the meaningful and multiple benefits of agroforestry planning from this understanding of landscape. Part II: Productive function of trees in the landscape

Chapters 9–16 provide a comprehensive discussion of how trees and shrubs can be used for productive purposes in southern Australia. The primary productive function of a tree is its wood, and the section begins with Rowan Reid’s ‘Wood as a farm product’. Land managers dealing with tree planting do not have to be foresters, but they do need to understand wood as a product for the trees to reach a market and for the landholder to receive a worthwhile return. Reid describes tree and wood growth and how we can manage intertree competition in a plantation to achieve the optimum yield and quality of marketable wood. With an explanation of log prices and harvesting costs, Reid outlines management principles that will achieve the best trees for different target products. Reid follows this in the next chapter, ‘Growing high-quality sawlogs’, in which he describes in greater detail the silviculture, value-adding and marketing of farm-grown sawlogs. Sawlogs are by no means the only wood product available from farms: firewood and pulp are two very important alternatives. Sawlog production has traditionally been restricted to higher-rainfall environments (>700 mm) and great advances are being made in extending its range into the lower rainfall zone (>400 mm). In Chapter 11, ‘Farm firewood production’, Peter Bulman and Ian Nuberg show how the relative value of firewood

versus sawlog production is strongly determined by the costs of production, changes in the value of the products and the value given to an early harvested product. Depending on the physical and economic context, firewood production can sometimes compete with sawlog production even under relatively high rainfall situations. In Chapter 12, ‘Pulpwood production’, Richard Harper, John McGrath, Keith Smettem, Rowan Reid and Andrew Callister describe the history and extent of this very important industry. Covering an area of approximately 500 000 ha in 2006, Tasmanian blue gum production for pulpwood is the largest form of farm forestry attracting significant private investment from urban sources. The chapter outlines the process of site selection and yield prediction, crucial elements of the planning process for blue gum establishment. It discusses the environmental, legal and marketing issues associated with blue gum production – understanding these is very important for farm foresters and land use planners, to ensure that the plantations are in the right place for the right reason. The chapter also includes an outline of the essential steps in the growing of blue gums. There are more than 400 million ha of grazing land in Australia, where trees can most readily be integrated with the agricultural system. In Chapter 13, ‘Trees in grazing systems’, Rowan Reid reviews our understanding of the impact of trees on pasture and animal production. Trees will compete with pasture just as they do with crops but the relative net effect on pastures is less severe. When the marked benefits of shade and shelter on animal production are included, along with the opportunity for wood production in higher-rainfall areas, trees can be easily justified on a farm financial basis. Reid illustrates this with a case study of integrating multi-purpose trees and grazing on a property in Victoria. About 1 million ha of Australia’s are in lowrainfall (300–450 mm) saline landscapes and may not be suitable for the trees and systems that Reid describes. Naturally occurring saltbush (Atriplex spp.) has long been the basis of the pastoral industry in the southern pastoral zone, as these plants have high levels of salt tolerance. Ed Barrett-Lennard and Hayley Norman describe the use of saltbush in Chapter 14, ‘Saltbush for fodder production on Saltland’. They describe the types of saltbush


that can be used for plantations as drought reserve or fodder on saline lands. When matching sites to pastures that include saltbush, it is important to understand the relationships between saltbush production and salinity, waterlogging and inundation. The chapter explains what can be expected from animal production and water use on saltbush pastures, and the key to their establishment. Fodder is not the only productive land use on saline lands and Nico Marcar explores this issue further in Chapter 15, ‘Productive use and rehabilitation of saline land using trees’. Salinity is one of the major forms of land degradation affecting agricultural lands, so Marcar outlines the problem’s extent and nature before explaining the range of native timber species’ genotypic response to salinity. This needs to be understood so that farm foresters can minimise the risks to growth when planting trees in saline landscapes. Marcar describes the main tree planting configurations being followed and how the economic opportunities in timber and non-wood products from these trees depend on growth rates, product quality and market considerations. He shares his thoughts on the prospects for the rehabilitation of saline lands with productive trees. This section on the integration of productive woody perennials into farming systems is capped off with a case study of mallee in the wheatbelt of Western Australia. John Bartle’s chapter, ‘Integrated production systems’, reviews the development of a short harvest cycle, mallee coppice system which produces oil, bioenergy and industrial wood and carbon products at the same time as controlling groundwater responsible for dryland salinity. It is a heroic task which, at the time of publication, is still not completed. The chapter shows the complexity and long development time involved in developing a new industry based on woody perennials. Part III: Implementation of agroforestry

This last section (Chapters 17–19) of the book discusses the essential points of implementing agroforestry: how to evaluate agroforestry, how government policy can facilitate agroforestry, and the adoption of agroforestry by landholders. David Thompson and Brendan George’s chapter, ‘Financial and economic evaluation of agroforestry’, outlines the essential principles and techniques of evaluating the financial feasibility of agroforestry systems as a farm business, and the

economic impact at the catchment level. It outlines a study of an agroforestry system, detailing the essential criteria and comparing farming options. This systematic financial and economic approach is useful for landholders to make decisions about investing in agroforestry. It is also essential for resource managers to be able to make economic measures of the environmental benefits from agroforestry. However, other criteria are of equal importance, including farmer intentions and capacity to operate. Some issues and opportunities are poorly addressed through economic considerations and market mechanisms do not always reflect where community and environmental benefits can be realised, allowing the government to intervene. David Pannell’s chapter, ‘Enhancing the environmental benefits of agroforestry through government policy mechanisms’, discusses the circumstances in which government intervention to enhance environmental outcomes from agroforestry is appropriate. He weighs the pros and cons of a wide variety of policy approaches and mechanisms, and concludes with the difficult question of who should pay for the public environmental benefits generated on private land. In the final chapter of this book, Digby Race discusses the adoption of agroforestry in Australia. He presents a historical and policy context determining adoption of agroforestry, with a focus on medium- to low-rainfall areas or the wheat-sheep zone, then outlines various extension strategies for enhancing adoption of agroforestry. The DVD

The DVD that accompanies this book offers the following material to complement and enhance the knowledge presented in the text. This material includes tools and instructions as well as inspirational stories. Videos

Tree measurement for farm forestry Yan Yan Gurt Farm Harvesting sawlogs at Bambra farm

Regional resources

Qld/NSW South Australia Tasmania Victoria Western Australia

Master Tree Growers

Design principles for agroforestry The Farmers Forest

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Greening Australia

Multiple benefits of farm forestry

Treesmart

Database of agroforestry species

Farm Forestry and Agroforestry Reference Library

Database of references to publications, non-published reports and research projects on farm forestry and agroforestry in Australia

Farm Forestry Toolbox

Collection of programs for assisting managers of shelterbelts, plantations or native forests

References ABARE (2007) Australian Farm Survey Results 2004–05 to 2006–07. Australian Bureau of Agricultural and Research Economics. Commonwealth of Australia. Abel N, Baxter J, Campbell A, Cleugh H, Fargher J, Lambeck R, Prinsley R, Prosser M, Reid R, Revell G, Schmidt C, Stirzaker R and Thorburn P (1997) ‘Design principles for agroforestry: a guide to assist farmers to decide where to place trees and farm plantations on farms’. RIRDC/LWRRDC/FEPRDC Joint Venture Agroforestry Program, Canberra. ABS (2008) Yearbook of Australia, 2008. Catalogue no. 1301.0. Australian Bureau of Statistics, Commonwealth of Australia. Alexandra J and Hall M (1998) ‘Creating a viable farm forestry industry in Australia: what will it take?’ Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. 98/74. Anderson LS and Sinclair FL (1993) Ecological interactions in agroforestry systems. Agroforestry Abstracts 6(2), 57–91. Aplin GJ (1998) Australians and Their Environment: An Introduction to Environmental Studies. Oxford University Press, Melbourne. Australian Government (2008) Carbon pollution reduction scheme: Australia’s low pollution future. Accessed 13 January 2009. Barr N (2000) Structural changes in Australian agriculture: implications for natural resource management. Department of Natural Resources and Environment, Victoria. Accessed 18 Janu-

ary 2008. . Barr N (2003) Future agricultural landscapes. Australian Planner 40, 123–127. Barr N and Wilkinson R (2005) Social persistence of plant-based management of dryland salinity. Australian Journal of Experimental Agriculture 45, 1495–1501. Biles L (2001) What America’s federal and state governments want from private forests: their preparedness to fund programs related to government being a stakeholder in private forestry. Paper presented at the IUFRO Working Party (S6.06–03) Symposium on Forestry Extension: Assisting Forest Owner, Farmer and Stakeholder Decision-making. 29 Oct–2 Nov, Lorne, Australia. . Bird PR, Kearney GA and Jowett DW (1994) ‘Trees and shrubs for south west Victoria’. Department of Agriculture, Pastoral and Veterinary Institute, Hamilton, Vic. Technical Report Series no. 205, February 2004. BRS (2007a) Australia’s Forests at a Glance. National Forest Inventory, Bureau of Rural Sciences. Commonwealth of Australia. BRS (2007b) Australia’s Plantations 2007: Inventory Update. Bureau of Rural Sciences. Commonwealth of Australia. Campbell A (1994) Landcare: Communities Shaping the Land and the Future. Allen & Unwin, Sydney. Cummine A (1999) Farm forestry: how is it different? Paper presented at the Institute of Foresters of Australia National Conference, Hobart. DAFF (1995) National Forestry Policy Statement: a new focus for Australia’s forests. December 1992, 2nd edn 1995. Department of Agriculture, Fisheries and Forestry, Commonwealth of Australia. Accessed 4 March 2008. . DAFF (2007) Australian Agriculture, Fisheries and Forestry at a Glance 2007. Department of Agriculture, Fisheries and Forestry, Commonwealth of Australia. Donaldson J and Gorrie G (1996) Farm forestry policy. In Proceedings of Australian Forest Growers Conference: Farm Forestry and Plantations


– Investing in Future Wood Supply. 9–12 September, Mt Gambier, SA. p.7. Dovers S & Wild River S (Eds) (2003) Managing Australia’s Environment. Federation Press, Sydney. FAO (2008) Agroforestry systems. Accessed 14 April 2008. . Garnaut R (2008) ‘The Garnaut climate change review’. Cambridge University Press, Cambridge. Hamnett S and Freestone R (eds) (1999) The Australian Metropolis: A Planning History. E & FN Spon, New York. Hopton H, Schmidt L, Stadter F and Dunkley C (2001) Forestry, land use change and water management: a green triangle perspective. In Proceedings of a National Workshop: Plantations, Farm Forestry and Water. 20–21 July 2000, Melbourne. (Eds EKS Nambier and AG Brown). RIRDC Publication no. 01/20. JVAP (2002) ‘Emerging products and services from trees in lower rainfall areas’. Joint Venture Agroforestry Program Research Update Series no. 2. Rural Industries Research and Development Corporation, Canberra. JVAP (2006) The Joint Venture Agroforestry Program: a partnership between Rural Industries Research & Development Corporation, Land and Water Australia, Forest and Wood Products Research & Development Corporation. . Kavanagh R, Law B, Lemckert F, Stanton M, Chidel M, Brassil T, Towerton A and Herring M (2005) ‘Biodiversity in eucalypt plantings established to reduce salinity’. Canberra. RIRDC Publication no. 05/165. Lundgren BO (1982) What is agroforestry? Agroforestry Systems 1(1), 7–12. NAFI (2006) Key statistics 2006: Australia’s forests, forest products, consumption trade and employment. National Association of Forest Industries. Accessed 14 February 2007. . Pannell D, Marshall G, Barr N, Curtis A, Vanclay F and Wilkinson R (2006) Understanding and promoting adoption of conservation practices by rural landholders. Australian

Journal of Experimental Agriculture 46(11), 1407–1424. Pannell DJ, Ridley A, Seymour E, Regan P and Gale G (2007) ‘Regional natural resource management arrangements for Australian states: structures, legislation and relationships to government agencies (April 2007)’. CRC for Plantbased Management of Dryland Salinity, Perth. SIF3 Working Paper 0701. Parsons M, Frakes I and Gerrand A (2007) Plantations and Water Use. Bureau of Rural Sciences, Canberra. Pearson C, Coakes S and Aslin H (2000) Social research to support successful farm forestry. In ‘Socio-economic research to support successful farm forestry’. Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. 01/13. Plantations 2020 (2003) Plantations for Australia: the 2020 vision. An industry/government initiative for plantation forestry in Australia. 2002 rev. edn. Accessed 4 march 2008. . Productivity Commission (2004) ‘Impacts of native vegetation and biodiversity regulations’. Melbourne. Report no. 29. Prosser M (1995) An industry perspective on farm forestry. Paper presented to the Graduate Certificate in Forest Science (Farm Forestry), University of Melbourne, Melbourne. Reid R and Stephen P (2001) The farmer’s forest: multipurpose forestry for Australian farmers. Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. R01/33. Reid R and Wilson G (1985) Agroforestry in Australia and New Zealand. Goddard & Dobson, Melbourne. RIRDC (1992) Proceedings of a Workshop on Low Rainfall Agroforestry. 30–31 August Yanchep, WA. Rural Industries Research and Development Corporation, Canberra. Schirmer J (2000) It’s not easy being green: perceptions of the 2020 Vision for plantation forestry in Australia. In Socio-economic Research to Support Successful Farm Forestry. Selected Papers and Abstracts from the ANU Forestry Colloquium. 28 February 2000, Canberra. (Ed. D Race) pp. 22–39.

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Schirmer J, Parsons M, Charalambou C and Gavran M (2005) ‘Socio-economic impacts of plantation forestry in the south west slopes of NSW, 1991 to 2004’. Report produced for FWPRDC Project PN04.4007. Forest and Wood Products Research and Development Corporation, Melbourne. . Senanayake R and Jack J (1998) Analogue Forestry: An Introduction. Department of Geography and Environmental Science, Monash Publications in Geography, no. 49. Monash University Press, Melbourne. Southern Cross Group (2006) A proposal for stewardship support to private native forests in NSW. . Spencer R, Bigsby H, Campbell A, Hoare J, Jellinek L, Reid R and Senanayake R (1989) ‘Consultant’s report to State Plantation Impact Study, May 1989’. Report prepared for the Steering Committee by the Centre for Farm Planning and Land Management, University of Melbourne, Melbourne. Stephens N (2001) The contribution of farmers and individual landholders to commercial planta-

tion development in Australia. Paper presented at IUFRO Working Party (S6.06–03) Symposium: Forestry Extension – Assisting Forest Owner, Farmer and Stakeholder Decision-making. 29 Oct–2 Nov 2001. . Stern NH (2007) ‘The economics of climate change: the Stern review’. Cambridge University Press, Cambridge. Walker RH (2000) The National Landcare Program: What has it Changed? A Compilation of Evaluations of the National Landcare Program, 1992– 1998. Department of Agriculture, Fisheries and Forestry, Canberra. Williams J and Saunders D (2002) Land use and natural ecosystems: a revolution in land use is the key to a sustainable landscape. Paper presented at Symposium: Getting it Right: What are the Guiding Principles for Resource Management in the 21st Century? March, Adelaide. Yamin F (Ed.) (2005) Climate Change and Carbon Markets: A Handbook of Emission Reduction Mechanisms. Earthscan, London. Yencken D and Wilkinson D (2000) Resetting the Compass: Australia’s Journey towards Sustainability. CSIRO Publishing, Melbourne.

PA R T I

Environmental function of trees in the landscape

2 Agroforestry and the functional mimicry of natural ecosystems EC Lefroy

This chapter examines the idea that agroforestry systems designed as structural and functional mimics of natural ecosystems could address the resource depletion and land degradation typical of conventional tillage agriculture. Four questions relating to this concept are examined. Is it necessary to mimic structure to achieve functional goals? Does perenniality inevitably imply a trade-off in productivity? Can competition be managed in synthetic polycultures of trees and crops? How can these complex farming systems be successfully managed to ensure a return to the farmer? It is concluded that, while natural ecosystems can serve as models for tighter cycling of water and nutrients, it is not necessary to assemble close structural mimics to achieve functional goals, and the altered soil and water conditions of most agricultural landscapes reduce the relevance of the pre-agricultural plant communities as models. Second, natural ecosystems tell us nothing about maximising harvestable product. We must turn to other strategies to ensure agroforestry systems are productive, notably artificial breeding and selection in the search for high-value products and services. Third, in mixtures of woody and herbaceous plants, competition tends to rule, particularly in the water-limited environments typical of southern Australia, and trees tend to win, requiring careful design and management of the interactions between trees and crops. Fourth, in terms of adoption and management, woody perennial systems are less flexible than conventional agriculture, have longer lag time to returns and higher investment

costs, and are less easily trialled. While these obstacles are not insurmountable, they collectively suggest that using agroforestry to mimic patch dynamics and exploit unused resources at landscape scale is more likely to achieve improved natural resource management in southern Australia than the smallerscale approach of mimicking the structure and function of natural plant communities at paddock scale.

Introduction The idea of looking to natural ecosystems as models for agriculture has a long and colourful history. Berry (cited in Jackson 1994) traces the history of nature as a measure or standard for agriculture back to Virgil over 2000 years ago, a theme later picked up by Spenser, Milton and Shakespeare. Alexander Pope in his Epistle to Burlington encouraged farmers to ‘Consult the genius of place in all’. As the environmental consequences of Australian agriculture have become more widely recognised, there has been increasing interest in consulting the genius loci, or spirit of this place, to understand the strategies that evolved within Australia’s native plant communities to enable them to survive on ancient leached soils in a highly variable climate. This chapter asks whether there are lessons for agroforestry in the study of native ecosystems, or whether the concept of nature as measured is best left with the Romantic poets. Agroforestry is one of several practices to have emerged over the last 50 years in response to resource 23

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degradation in farming systems (Table 2.1). In his book Tree Crops: A Permanent Agriculture, first published in 1958, Smith (1987) argued that the problem of agriculture undermining its resource base began when the agriculture of the plains was carried to the hills 8000–9000 years ago. His solution was to leave the alluvial soils of the valleys under annuals and farm the hill slopes with tree crops. Archaeological studies of the hills around Athens have revealed a sequence of settlement, erosion and abandonment, followed by deposition, then resettlement, erosion and abandonment repeated over hundreds of years. Pre-empting Smith by 2400 years, the Athenian politician Peisistratus argued for a law that made lowinterest loans available to hill farmers to replace their annual crops with vines and olives. Smith’s solution for the continental US was to prescribe agroforestry species suited to each climatic zone. A common feature of the farming systems listed in Table 2.1 is that their boundary of concern has extended beyond paddock and farm to consider the wider impacts on the environment (Figure 2.1). It is only over the last two decades that scientific inquiry in southern Australian agriculture has extended deeper than the root system of annual crops, outside the winter growing season and beyond the farm boundary. This emerging interest in ecological sustainability and the environmental impact of agriculture is neatly captured in Bawden and Sriskandarajah’s (1993) four phases of Australian agriculture. In the pioneering phase, the emphasis was on settlement and food self-sufficiency for the early colonies. By the turn of the 20th century, improved understanding of plant nutrition and fertilisers led to an emphasis on maximising producTable 2.1: Agricultural systems emerging over the last 50 years in response to resource depletion and degradation Farming system

Reference

Permanent agriculture

Smith (1958, 1987)

Natural systems agriculture

Jackson (1985)

Agroforestry

Nair (1987)

Holistic resource management

Savory (1989)

Agroecology

Altieri (1990)

Permaculture

Mollison (1990)

Figure 2.1: A hierarchical view of farming systems showing constraints at each scale.

tion at paddock scale. Declining terms of trade after the Second World War led to a necessary emphasis on efficiency and productivity and saw the rise of farm management groups. In the last decades of the 20th century, recognition of the threat posed by salinity, soil acidity and other land degradation issues, together with the emergence of the environmental movement and the discipline of landscape ecology, saw the boundaries of consideration and inquiry move to catchment and regional scales. Three of the farming systems in Table 2.1 have been strongly influenced by observation of natural ecosystems. In the case of Savory (1989), observations of the grazing patterns of native African herbivores and an appreciation of the need to match grazing pressure with the phenology of pasture species (Voisin 1988) led to the development of holistic resource management. Permaculture was influenced by the recognition that perenniality and diversity were common features of natural systems (Mollison 1990), and these features were adopted as design criteria for sustainable food production. However, it is Jackson and co-workers (Jackson and Bender 1984; Jackson 1985; Jackson and Piper 1989; Jackson 1994; Jackson and Jackson 2000) who have pursued the idea of ecosystem mimicry to the greatest extent through their work since the mid 1980s, aimed at assembling high seed-yielding perennial

2 – Agroforestry and the functional mimicry of natural ecosystems

polycultures as mimics of native grasslands. Jackson’s model of sustainability was the mid-grass prairies of the Great Plains. He saw that wheat fields on sloping ground caused soil erosion beyond replacement levels and that farming them depended on fossil fuels and applications of chemicals with which humans have had no evolutionary experience. Alongside these, in comparison, were areas of never-ploughed native prairie running on sunlight, self-sufficient in nitrogen and making full use of rainfall (Jackson 1993). Implicit in Jackson’s prairie model is the assumption that the structure of natural ecosystems can serve as a guide to achieving the functional goals of sustainable agriculture, namely tighter cycling of water and nutrients and improved energy use efficiency. As the original plant communities that preceded most of southern Australia’s farm lands were woodlands and heath, our model for sustainable agriculture under the mimic concept becomes agroforestry. This chapter explores four implications of the mimic concept with respect to agroforestry in southern Australia. How closely do we need to mimic structure to achieve functional goals? Does perenniality inevitably imply a trade-off in productivity? Can competition be managed in synthetic polycultures of trees and crops? How can complex farming systems be successfully managed?

Implications of the mimic concept Mimicking structure to achieve functional goals

One of the major challenges facing agriculture in southern Australia is to tighten water and nutrient cycles. Leakage of water results in rising watertables, waterlogging and salinity. Leaching of nutrients below the root zone of annual crops and pastures results in pollution of ground and surface waters. One of the pioneering studies of the relationship between agroecosystem structure and function was the work of Jack Ewel in Costa Rica. After many years of visiting Costa Rica, Ewel observed that the rotation length of its slash-andburn farming system was being shortened due to increased food demand. As the cycle shortened, fewer nutrients were held in the standing forest at

the time of clearing to be made available through the ash bed to subsequent crops. Yields declined, soil erosion and leaching of nutrients increased and a cycle of degradation set in, a pattern common in tropical forests around the world. Ewel was interested in the design of a permanent agriculture as a way out of the cycle. In the mid 1970s he designed an experiment that involved taking a patch of recently cleared and burnt forest and imposing five different treatments. In the first, the natural successional processes were allowed to take place. In the second, the traditional farming practice of one or two crops of maize followed by cassava then regeneration was carried out. In the third, the land was continuously cropped. The fourth treatment was his template for a permanent agriculture: natural succession was mimicked using only exotic plants, substituting tree for tree, shrub for shrub and vine for vine with the aim of having the same representation of life forms. In the fifth treatment, natural succession was augmented with the exotic plants used in the fourth treatment. Eventually the economic potential of these exotic substitutes was to be investigated, but in the first instance it was the functional implications of tighter water and nutrient cycling that interested Ewel. Ewel and co-workers studied the fate of water and nutrients under each of these management regimes. Their conclusion was that soil-nutrient dynamics similar to the successional vegetation could be achieved in a human-built mimic. The implications of this finding for agroecosystem design are immense: create the structure and the nutrient retention will follow. This opens the way for substitution of species that are particularly desirable from economic, conservation or aesthetic perspectives, into complex systems (Ewel et al. 1991). Pate and Bell (1999) applied this concept of mimicking natural ecosystems to an isolated Banksia woodland community in south-western Australia, in a landscape where the original vegetation had been extensively cleared and replaced by cereal–legume crop rotations and legume-based pastures. Two consequences of this development were rising watertables and nitrate pollution of groundwater. Excavation of above- and belowground biomass of 45 of the most common woody

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Figure 2.2: Two models for an ecosystem mimic of a Banksia woodland. (a) A restoration ecology model (Character 3) requiring eight plant types based on root morphology and seasonality of growth (deep-rooted summer active and shallow-rooted winter active) life form (tree and shrub) and fire response (seeders and re-sprouters). (b) A productive model requiring four plant types (Character 2). Numbers indicate species found in each class at the study site (modified from Pate and Bell 1999).

plant species, plus sampling of understorey herbaceous plants, revealed that trees accounted for 69% of the total biomass, shrubs 24%, herbaceous perennials 6.5% and herbaceous annuals less than 1%. From studies of root morphology, phenology, trophic specialisations and fire response, they developed criteria for selecting the composition of ecosystem mimics for two scenarios: a restoration ecology model where the mimic system replaces cleared virgin woodland not subject to incursions of water and nutrients from surrounding agriculture, and a production model designed to profitably rehabilitate agricultural land experiencing rising watertables and nitrate pollution. The second scenario highlights a limitation to the use of nature as model: over much of Australia’s agricultural lands the initial conditions to which the native plant communities were so well-adapted no longer prevail, so an alternative model for sustainable agriculture must be used to suit the altered water and nutrient regimes.

For the first scenario (replacing cleared virgin woodland) Pate and Bell (1999) applied a fourstage species selection process based on growth phenology, root morphology, life form and fire response (Figure 2.2). The result is a minimum set of eight plant types required for a functional mimic. Adding four trophic specialisations typical of this woodland community (proteoid roots, mycorrhizal types, symbiotic nitrogen fixation, carnivorous and parasitic habit) elevates that to 32. Allowing for redundancy to compensate for establishment failure, the required number of species for a self-sustaining mimic rises to over 100. This replacement scenario is based on a literal interpretation of structural and functional mimicry for the purposes of rehabilitation with no consideration given to productive capacity or harvest. It highlights the potential complexity of developing self-sufficient functional mimics. If we exclude fire and supply nutrients to replace those removed through harvest of commercial woody


species, we have a productive model based on a minimum of four plant types (Figure 2.2, Character 2). Here, management substitutes for diversity of form and function. Pate and Bell (1999) effectively describe strong and weak forms of the mimic concept. The similarity of the weak version to the pre-agricultural community ends with the prescription for a predominance of summer-active, deep-rooted woody species. The differences are a need for faster growth rates to make inroads into elevated watertables, planting layouts that can achieve higher utilisation of groundwater per unit area than experienced in annual crops or native vegetation, and species capable of stripping excess nitrate and other nutrients from groundwater. Two caveats apply to these design rules. First, if woody legumes are used, when cut or grazed they typically make a greater net contribution to system nitrogen than they take up from soil and groundwater (Unkovich et al. 2000). Second, nutrient-stripping by deep-rooted species will only be effective if their biomass is removed from the site, for example as grain or timber. A test of the Pate and Bell (1999) weak mimic comes from two studies at adjacent sites that examined the potential of perennial farming systems to achieve similar water balance to that of native vegetation. At an 8 ha site that had been cleared of Banksia woodland and farmed for 40 years, Lefroy et al. (2001) compared the components of the water balance in annual cropping, an alley cropping system with 20% tree cover, and a dense plantation. The tree species was the forage legume tagasaste (Chamaecytisus proliferus Link.) which was cut the previous year and allowed to regrow. A perched watertable had formed, rising to within 5 m of the surface and with a nitrate concentration of 1 ppm N. They found that the spaced agroforestry system was capable of significantly reducing deep drainage below the root zone of crops compared to annual cropping (32 mm and 193 mm respectively; Table 2.2, Figure 2.3). The plantation trees transpired at a rate equivalent to 2.3 times annual rainfall, resulting in uptake of 600 mm from groundwater in one year. The rapid growth of the trees meant that annual net primary productivity of the plantation was 18 t/ha–1 of above-ground biomass, three to four times that of annual crops and six times that of the Banksia woodland (Pate and Bell 1999).

Table 2.2: The fate of rainfall under annual crop, tagasaste (Chamaecytisus proliferus) alley crop and tagasaste plantation (371 mm December 1997– December 1998)

Crop

WUa

Sole crop

Alley crop

Plantation

234

194

–

Tree WU (soil)

–

45

236

Tree WU (GW) b

–

122

599

Interceptionc

–

28

140

Soil water

–65

-50

0

Drainage

193

32

–604

a: Water use b: Groundwater c: Rainfall intercepted by the tree canopy Source: Lefroy et al. (2001)

However, the heavy dependence of the tagasaste plantation on groundwater raises questions about the sustainability of that system if extensively planted. The hydrology of the area dictates that the right balance would need to be struck between the area under annuals supplying groundwater recharge, the area established to plantations and the discharge capacity of the groundwater system. The trees in the experimental plot are likely to succumb to rising groundwater, given the small scale of the plantation. This balance emphasises how far the system has been shifted from its preagricultural state and the limitations of the mimic concept in such altered environments. In managing water in agricultural landscapes, the boundary of consideration moves up to the catchment level, whereas the focus of the mimic concept as articulated by Jackson and Ewel has been on plant community structure at the patch or paddock scale. In this case, the higher-order constraints dominate the question of what constitutes sustainable land use.

Figure 2.3: Root systems of sole crop, alley crop and plantation.

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Ward et al. (2003) tested the effectiveness of a herbaceous perennial plant community in managing water in the same environment by comparing water budgets for the Banksia woodland community studied by Pate and Bell (1999) with an adjacent area of introduced pasture (Medicago sativa L.). In its establishment year, a one-in-50 high rainfall year for the region, drainage under the lucerne was measured at 180 mm compared to 80 mm under the Banksia woodland. The following year, the cumulative evapotranspiration of the lucerne was greater than that of the Banksia woodland, approaching that of potential ET, and the lucerne dried the soil to a depth of 4.5 m, the same extent as under the woodland. Both lucerne and woodland plant communities effectively controlled excess water leakage, indicating that very different plant community structures and composition can be used to arrive at the same functional endpoint. The lucerne pasture, however, is wholly dependent on one species and is therefore more vulnerable to disturbance through disease, herbivory or weather extremes. The woodland features considerable redundancy through complementary patterns of growth and water use among a large number of species. Significantly, in this case a herbaceous perennial monoculture achieved the same result as a woody perennial polyculture, indicating that it is not necessary to mimic structure to be granted function, at least for water use. The test will be in the longevity and robustness of the lucerne system.

Perenniality and the trade-off between production and persistence Developing production systems that mimic the structure and function of natural ecosystems almost inevitably implies relying on the harvest of perennial plants. This presents two significant challenges. First, it challenges the principle of biology that plants partition their resources towards reproduction or perenniality, but not both, leaving a smaller proportion of annual growth available for harvest. In water-limited environments such as the Mediterranean-type climate experienced over much of southern Australia’s agricultural regions, survival over summer for most plant communities requires investment in permanent woody structure

above- and below-ground. This is necessary to access water and nutrients at depth in ancient leached soils with low water-holding capacity. Second, it flies in the face of history as the success of agriculture for over 10 000 years has been primarily based on fitting high-yielding annual plants into short seasonal windows to avoid climatic extremes. In southern Australia, where replacement of predominantly woody ecosystems by synthetic annual grasslands has had dramatic consequences for water and nutrient cycles, the question is whether it is possible to have a permanent form of agriculture with acceptable levels of production. A measure of the investment in structure required to persist in a Mediterranean-type climate on infertile leached soils comes from the work of Pate and Bell (1999) and Lefroy et al. (2001), who measured net primary productivity of a Banksia woodland, annual crops and pastures, and a coppiced tagasaste plantation (Figure 2.4). Annual net primary productivity of the Banksia woodland amounted to only 3 t/ha–1, or some 11% of the total biomass of 32 t/ha–1. The harvest index of ~10% is typical of that reported for forest systems harvested annually for products such as latex or fruit (Ewel 1999). The cost of persistence is high: 64% of total biomass is invested in permanent stems and roots to ensure access to scarce water and nutrients through very deep roots and trophic specialisation. Annual crops grown on the same soil, but with additions of phosphorus and nitrogen, produced a total annual biomass of 10 t/ha–1 with a harvest index of 50%, despite the fact that up to 40% of annual rainfall is typically unused and drains below the root zone. Coppiced tagasaste, by comparison, had a net annual productivity of 18 t/ha–1 from an investment of 8 t/ha–1 in belowground biomass and 2 t/ha–1 in above-ground biomass (the cut stump). There is a dramatic contrast between the near-climax community of the Banksia woodland and the relatively young tagasaste trees, cut to within 60 cm of ground level the previous year and exhibiting extremely fast growth rates as they restored root:shoot biomass ratios to predisturbance levels. This comparison suggests that a strategy to minimise the trade-off between permanent structure and productivity in this environment is to keep the woody component in an active growth stage through


Figure 2.4: Annual net primary productivity in Banksia woodland, coppiced tagasaste plantation and annual crops and pastures grown on deep infertile sands near Moora, WA (based on data from Pate and Bell 1999; Lefroy et al. 2001).

regular disturbance. However, this has implications for competition (discussed below). Other strategies to compensate for low levels of production involve finding high-value products or markets for ecosystem services rather than physical products. The most effective strategy, however, is domestication. Selection and breeding of fruit trees has resulted in dry matter yields comparable with those of annual crops, with average US yields in oranges, apples and pears of 2.5, 3.8 and 5.4 t/ha–1 dry matter respectively (less peel and core), compared to average wheat yields of 2.5 t/ha–1 (Jackson and Jackson 2000). Jackson and Jackson (2000) suggest that the differences between annual and perennial crops are neither as great nor as fundamental as they seem, and that maintaining high yield in woody perennials may present fewer challenges than in herbaceous perennials. They argue that, in both annuals and perennials, the challenge is to produce a canopy as rapidly as possible to accumulate the products of photosynthesis. In both cases, this requires meristems and start-up energy. In the annual, the meristems are the seeds in which the start-up energy is stored internally. In perennials, the meristems are the dormant buds and the start-up energy is stored in roots and shoots. In woody perennials, the number and distribution of meristems can be controlled through annual pruning; in herbaceous perennials there is

the greater challenge of maintaining the density of meristems over time. In herbaceous perennials there is evidence that perenniality and high seed yield may not be mutually exclusive. Research on a native American grass, Tripsacum dactyloides (eastern gamagrass or sesame grass) showed that high seed yield (>1 t/ ha) in a pistillate mutant did not compromise its perenniality (Jackson and Dewald 1994; Piper 1998). Additional evidence comes from a perennial wheat-breeding program at the University of California at Davis in the 1940s that produced intergeneric hybrids between wheat (Triticum aestivum) and a perennial relative, Agropyron ponticum. Lines bred specifically for perennial habit yielded within 70% of the best commercial wheats of their time (Suneson and Pope 1946). In summary, while natural ecosystems can serve as useful models for tighter cycling of water and nutrients, they tell us little about maximising a harvestable product. Maximising total biomass productivity in woody perennials can be achieved through high levels of disturbance to keep stands in vegetative growth. Maximising reproductive harvest requires breeding, selection and more selective disturbance. There is encouraging evidence that the obstacles to high yield in perennials are neither as great nor as fundamental as they seem, but a major challenge is the proportion of

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growth that plants must allocate for survival in water-limited environments.

Competition rules in early-stage succession Valuable insights into competition in agroforestry have come from studies of alley farming in Africa. Alley cropping, a practice designed to reduce soil erosion and nutrient depletion in tropical and subtropical cropping systems, was widely promoted in Africa and south-east Asia in the 1980s (Kang et al. 1992). On adoption by farmers, however, it proved to be disappointing. Subsequent studies found that below-ground competition between trees and crops for water frequently outweighed any benefits to crops through soil fertility and microclimate, and increased the likelihood of crop failure. This finding seemed at odds with the ecological literature which reported that grassland benefited from proximity to trees. The productivity of grasses under mature savannah trees increased as rainfall decreased, with the tree canopy providing a beneficial niche for herbaceous species (Belsky 1994). This positive influence of spaced or parkland trees on understorey species had also been observed in the Mediterranean Dehesa system of southern Spain and Portugal, where production of cork and acorns from widely spaced oak trees (Quercus suber) has been managed in conjunction with grazing by sheep, cattle and pigs for over 800 years (Joffre and Rambal 1993). In reviewing this apparent contradiction, Ong and Leakey (1999) offered an explanation based on the different successional stages represented by young agroforestry systems and mature savannahs or open woodlands (Figure 2.5). As mature trees have a higher proportion of woody above-ground structure to foliage, more water is saved through

Agroforestry

Savannah and Dehesa

Figure 2.5: Competition rules in early-stage succession. A comparison between young agroforestry systems and mature trees in tropical savannahs (after Ong and Leakey 1999).

reduced soil evaporation under the canopy than is lost through transpiration. This leaves understorey plants with better water relations than those in the open. Joffre et al. (1999) found that in the Dehesa system tree-induced modifications in soil properties improved the availability and uptake of water for plant growth under the canopy compared to the areas outside the canopy. By contrast, high densities of young fast-growing trees with a high leaf:stem ratio are more likely to compete with crops for water and nutrients. Where selection of leafy species and frequent pruning are important for production, as in forage and other biomass-based agroforestry systems, competition is likely to be maintained. To achieve the advantages of mature systems requires 20–40 years, well beyond the planning horizon of most farmers (Ong and Leakey 1999). The fundamental premise on which agroforestry is based is that trees can exploit resources unavailable to crops, resulting in an improved environment for crops and greater production per unit area than would be possible under sole cropping (Sanchez 1995). The work of Ong and Leakey (1999) and others (Cooper et al. 1996; Van Noordwijk and Ong 1996; Rao et al. 1998) in reviewing interactions in agroforestry systems has questioned this assumption, pointing out that tree and crop roots are likely to occupy similar soil zones, and that indirect benefits of trees through shelter are likely to be outweighed by competition. Ong and Leakey (1999) concluded that, in water-limited environments, below-ground competition is inevitable and crops tend to lose, with crop failure more likely in agroforestry than sole cropping in very dry years. Clear benefits of shelter from trees have been demonstrated in cropping systems in more humid and fertile environments in the northern hemisphere (Brandle and Kort 1988). In Australia, a national study with data from four sites and multiple crops over three years, modelled using 30 years of climate data, suggested neutral to very slightly positive returns from planting windbreaks (Carberry et al. 2002). More positive response from shelter has been measured in grazing systems (Bird 1998). Several strategies can be employed to overcome competition between trees and crops. One is to select tree species of equal or higher value than the


crop, to reduce the importance of competition. Another is to manage competition at the tree–crop interface through root pruning. A third is to minimise competition through careful selection of site and species. Oliver et al. (2005) identified several site and system characteristics that are more likely to produce positive impacts on crop yield. In a study of 21 alley cropping sites in Western Australia and New South Wales, they found that systems featuring trees more than 10 years old, established on water-gaining sites (with access to runoff or shallow groundwater), on heavier soils and with an aspect that protected crop from the south and west, were more likely to result in yield increases in adjacent crops than other combinations. However, the highly variable nature of the climate in southern Australia meant that even the most favourable combinations of site and species showed a negative response in a dry year, through competition. Other studies have found that yield response to shelter from trees is more likely with legume crops than cereals (Bicknell 1991; Nuberg et al. 2002). A fourth strategy is to use temporal complementarity to avoid competition. Temporal complementarity involves selecting tree species that are dormant when the crop is growing and active when the crop is absent. This strategy will only work if there is residual water left after the crop or if rainfall occurs outside the cropping season. One of the few clear cases of temporal complementarity is the Sahelian tree Faiderbia albida (Poschen 1986; Ong et al. 1992). The use of deciduous trees in southern Australia would appear to fit this strategy, but competition in spring when crops are finishing and the trees are coming into their active phase is likely to result in competition, particularly in dry years. A fifth strategy is to use spatial complementarity. This involves placing trees in landscape niches where resources accumulate, and avoiding direct interaction with crops. In terms of the mimic concept, this means mimicking the patch dynamics commonly observed in natural ecosystems, rather than plant community structure at the patch scale. In summary, in water-limited environments, competition rules in tree–crop systems and the trees tend to win. While careful design of integrated systems such as alley cropping can reduce competition, using agroforestry to mimic patch dynamics and exploit unused resources at land-

scape scale is likely to be a more promising approach to achieving improved natural resource management than mimicking plant community structure at paddock scale.

Managing perenniality and competition The first and most significant challenge to the management of agroforestry systems, regardless of their environmental benefits, is to develop forms that are sufficiently profitable to warrant the interest of farmers. Where they are profitable, natural resource management outcomes are likely to be achieved as beneficial side effects of commercial adoption. The preceding discussion highlights two main challenges to the profitability of agroforestry systems: the low levels of productivity inherent in perennial and particularly woody systems, and the effects of competition on the reliability of crop and pasture production. Identifying high-value woody species and strategies to minimise or eliminate competition are therefore high-priority areas for research and development. Even where those conditions are met, inherent properties of agroforestry systems present challenges to their adoption and management. Compared to other agricultural innovations they are inflexible, slow to establish and produce return, difficult to try on a small scale and have high upfront costs (Pannell 1999). One of the great strengths of dryland agriculture in southern Australia has been the flexibility of mixed crop and livestock farming, and the ability to shift emphasis between enterprises and products in response to markets and climate. Agroforestry systems commit land to one form of use for long periods and thus can reduce the farmer’s ability to manage variability and risk. On the other hand, they can increase capacity to manage variability and risk where they result in net benefits to crop and pasture production through improved microclimate or soil conditions (e.g. erosion or waterlogging). In general, these effects have been clearly demonstrated only in the higher- rainfall margins of Australia’s agricultural regions. The difficulty of trialling an agroforestry system presents a particular challenge, as first-hand experience is important to a farmer’s decision to adopt

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Figure 2.6: Agroforestry mimicking large-scale patch dynamics at landscape scale, showing a mixture of Eucalyptus globulus plantations, annual pasture and remnant vegetation near Albany, WA (photo by CALM).

an innovation. This is particularly so where the value of the system lies in its ability to reduce a degrading process such as salinity or soil erosion – the promise of an uncertain outcome has to be weighed up against the certainty of capital costs and opportunity costs. However, the up-front costs involved in agroforestry systems may sometimes work to their advantage. Blue gum (Eucalyptus globulus) agroforestry experienced impressive growth during the 1990s with over 100 000 ha established on farms in southern Australia. This was only possible through share farming schemes in which initially governments and later the private sector formed partnerships with landowners, sharing costs and risks.

Conclusion Many of the problems of agriculture that led to the development of alternative farming systems stem from the homogenisation of the landscape (Jackson

1994). Land has been divided on the cadastral grid with any parcel treated much the same way as any other. Some parts of the landscape leak water and nutrients more than others, some are more prone to erosion. The ecosystem mimic concept is a reaction to this, that proposes a radical overhaul of farming systems and redesign at the patch scale using the structure and function of natural ecosystems as the template or model. This concept has a long history in its general form but only recent application in its specific form. That recent history has been in what Ewel refers to as a forgiving environment for agriculture (Ewel 1999), neither too cold, too dry nor too wet, and with a young substrate of deep soil left by glaciers that receded only 13 000 years ago. This chapter examined the ecosystem mimic concept from an Australian perspective. It addressed four questions: the extent to which we need to mimic the structure of natural ecosystems to achieve the functional goals of tighter water and nutrient cycling; whether there is a trade-off


between perenniality and productivity and how might we get around it; how we manage competition in agroforestry systems; and how we could address the challenges to adoption and management of complex farming systems based on perennial, especially woody perennial, plants. On the question of structure we saw that there are weak and strong interpretations of the mimic concept, depending on whether our goal is restoration or production, and that the altered conditions experienced in agricultural landscapes reduce the relevance of nature as model. We also saw, in the comparison between water use in lucerne and a Banksia woodland, that we don’t need a structural analogue to achieve functional goals. On the question of productivity in perennials, we saw evidence of the substantial investment in permanent structure required to persist in water-limited environments, but also encouraging evidence of strategies to increase and maintain high levels of productivity in managed perennial systems. On the question of competition, we saw the suggestion that the complementarity sought between the woody and herbaceous components in agroforestry systems is more likely to be achieved with mature trees. Taken together, these observations suggest that rather than attempting to mimic natural systems at the paddock or plant community scale, we should lift our boundary of consideration and examine where and how agroforestry systems can exploit unused resources at the landscape scale. Evidence of this landscape ecological view of agriculture can already be seen in practices such as farming to soil type, precision agriculture and mosaic farming, which represent a reintroduction of heterogeneity into the landscape based on an appreciation of place. This follows the spirit of Pope’s advice: ‘Consult the genius of place in all’. Improved natural resource management in southern Australia is more likely to come from mimicking patch dynamics at landscape scale rather than plant community structure at paddock scale.

Acknowledgements This chapter draws on research funded by Land and Water Australia’s program ‘Redesigning Australian Agricultural Landscapes’, the Grains Research and

Development Corporation, the Rural Industries Research and Development Corporation and the Joint Venture Agroforestry Program, and the Agricultural Landscapes Program within CSIRO Sustainable Ecosystems. Christine Davies prepared the figures and tables.

References Altieri MA (1990) Agroecology. Springer-Verlag, Berlin. Bawden RJ and Sriskandarajah N (1993) National perspectives in transition: the case of dryland salinity. In Proceedings of a National Conference on Land Management for Dryland Salinity Control. La Trobe University, Bendigo, Vic. (Eds JM Taylor and CA Clifton) pp. 13–17. Belsky AJ (1994) Influence of trees on savannah productivity: test of shade, nutrients and tree grass competition. Ecology 75, 922–932. Bicknell D (1991) The role of trees in providing shelter and controlling erosion in the dry temperate southern agricultural area of Western Australia. In Proceedings of A National Conference on the Role of Trees in Sustainable Agriculture. Rural Industries Research and Development Corporation, Canberra. pp. 21–39. Bird PR (1998) Tree windbreaks and shelter benefits of pasture in temperate grazing systems. Agroforestry Systems 41(1), 35–54. Carberry PS, Meike H, Poulton PL, Hargreaves JNG, Snell AJ and Sudmeyer RA (2002) Modelling crop growth and yield under the environmental changes induced by windbreaks. 2. Simulation of potential benefits at selected sites in Australia. Australian Journal of Experimental Agriculture 42, 887–900. Cooper PJM, Leakey RRB, Rao MR and Reynolds L (1996) Agroforestry and the mitigation of land degradation in the humid and sub-humid tropics of Africa. Experimental Agriculture 32, 235–290. Ewel JJ (1999) Natural systems as models for the design of sustainable land use. Agroforestry Systems 45, 1–21. Ewel JJ, Mazzarino MJ and Berish CW (1991) Tropical soil fertility changes under monocultures and successional communities of different structure. Ecological Applications 1, 289–302.

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Jackson W (1985) New Roots for Agriculture. University of Nebraska Press, Lincoln, Nebraska. Jackson W (1993) Meeting the expectations of the land. In Proceedings of a Workshop: Visions for Agriculture. (Ed. EC Lefroy). Merredin Dryland Research Institute Research Advisory Committee, Department of Agriculture, WA. Miscellaneous Publication 6/93. Jackson W (1994) Becoming Native to this Place. University of Kentucky Press, Kentucky. Jackson W and Bender M (1984) Investigations into perennial polyculture. In Meeting the Expectations of the Land. (Eds W Jackson, W Berry and B Colman) pp. 183–194. North Point Press, San Francisco. Jackson L and Dewald C (1994) Predicting evolutionary consequences of greater reproductive effort in Tripsacum dactyloides, a perennial grass. Ecology 75(3), 627–641. Jackson W and Jackson L (2000) Developing high seed yielding perennial polycultures as a mimic of mid-grass prairie. In Agriculture as a Mimic of Natural Ecosystems. (Eds C Lefroy, RJ Hobbs, MH O’Connor and JS Pate) pp. 1–37. Kluwer, Dordrecht. Jackson W and Piper J (1989) The necessary marriage between ecology and agriculture. Ecology 70, 1591–1593. Joffre R and Rambal R (1993) How tree cover influences water balance of Mediterranean rangelands. Ecology 74, 570–582. Joffre R, Rambal R and Ratte JP (1999) The dehesa systems of southern Spain and Portugal as a natural ecosystem mimic. Agroforestry Systems 45, 57–79. Kang BT, Reynolds L and Atta-Krah AN (1992) Alley farming. Advances in Agronomy 43, 315–359. Kort J (1988) Benefits of windbreaks to field and forage crops. Agriculture, Ecosystems & Environment 22–23, 165–190. Lefroy EC, Pate JS and Stirzaker RJ (2001) Growth, water use efficiency and adaptive features of the tree legume tagasaste (Chamaecytisus proliferus Link) on deep sands in south-western Australia. Australian Journal of Agricultural Research 52, 221–234. Mollison B (1990) Permaculture: A Practical Guide for a Sustainable Future. Island Press, Washington DC.

Nair PKR (1987) AgroforestrySsystems in Major Ecological Zones of the Tropics and Subtropics. International Council for Research in Agroforestry, Nairobi. Nuberg IK, Mylius SJ, Edwards JM and Davey C (2002) Windbreak research in a South Australian cropping system. Australian Journal of Experimental Agriculture 42, 781–795. Oliver YM, Lefroy EC, Stirzaker R and Davies C (2005) Deep drainage control and yield: the trade-off between trees and crops in agroforestry systems in the medium to low rainfall areas of Australia. Australian Journal of Agricultural Research 56, 1011–1026. Ong CK and Leakey RRB (1999) Why tree–crop interactions in agroforestry seem at odds with tree–grass interactions in tropical savannahs. Agroforestry Systems 45, 109–129. Ong CK, Odongo JCW, Marshall F and Black CR (1992) Water use of agroforestry systems in semiarid India. In Proceedings of the International Symposium: Growth and Water Use of Plantations. 7–11 February, Bangalore. (Eds IR Calder, LR Hall and PG Adlard) pp. 347–358. Wiley, Chichester. Pannell DJ (1999) Social and economic challenges in the development of complex farming systems. Agroforestry Systems 45(1–3), 393–409. Pate JS and Bell TL (1999) Application of the ecosystem mimic concept to the species-rich Banksia woodland of Western Australia. Agroforestry Systems 45, 303–341. Piper JK (1998) Growth and seed yield of three perennial grains within monocultures and mixed stands. Agriculture Ecosystems & Environment 68(1–2), 1–11. Poschen P (1986) An evaluation of the Acacia albida-based agroforestry practices in the Hararghe highlands of eastern Ethiopia. Agroforestry Systems 4, 31–56. Rao MR, Nair PK and Ong CK (1998) Biophysical interactions in tropical agroforestry systems. Agroforestry Systems 38, 3–49. Sanchez PA (1995) Science in agroforestry. Agroforestry Systems 30, 5–55. Savory A (1989) Holistic Resource Management. Island Press, Washington DC. Smith JR (1987) Tree Crops: A Permanent Agriculture. Island Press, Washington DC.


Suneson CA and Pope WK (1946) Progress with Triticum x Agropyron crosses in California. Journal of the American Society of Agronomy 38, 956–963. Unkovich MJ, Pate JS, Lefroy EC and Arthur DJ (2000) Nitrogen isotope fractionation in the fodder tree legume tagasaste (Chamaecytisus proliferus) and assessment of N-2 fixation inputs in deep sandy soils of Western Australia. Australian Journal of Plant Physiology 27(10), 921–929.

Van Noordwijk M and Ong CK (1996) Lateral resource flow and capture: the key to scaling up agroforestry results. Agroforestry Today 7, 29–31. Voisin A (1988) Grass Productivity. Island Press, Washington DC. Ward PR, Fillery IRP, Maharaj EA and Dunin FX (2003) Water budgets and nutrients in a native Banksia woodland and an adjacent Medicago sativa pasture. Plant and Soil 257, 305–319.

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3 Using trees to manage local and regional water balances Keith Smettem and Richard Harper

In this chapter we review some of the key aspects to consider when planning revegetation strategies that use trees to manage local and regional water balances. We commence by reviewing how trees use water and how they respond to environmental conditions, including drought, waterlogging and salinity. Understanding these responses are critical when selecting suitable species for managing the water balance in specific landscape locations. After providing definitions of typical groundwater systems we introduce the principle of ecological optimality and use this to explore available design options to manage the water balance using trees in dryland catchments (typically, 300– 600 mm mean annual rainfall). We conclude that the prospects for lowering watertables by revegetation with perennial vegetation would appear to be best in local groundwater systems, or where annual rates of groundwater inflow are considerably less than annual transpiration losses.

Introduction Across much of the cropping region of Australia clearing of predominantly perennial, deep-rooted native vegetation and replacement by shallow-rooted annual crops and pastures has resulted in a changed hydrological regime, with decreased transpiration leading to increased runoff and increased recharge to groundwater systems (Hatton and Nulsen 1999). In consequence, groundwater systems are rising,

mobilising regolith salt stores and leading to secondary dryland salinity on a massive scale (Beresford et al. 2001; Bari and Smettem 2006a). Controlling the spread of dryland salinity requires hydrologically appropriate management systems to be developed and implemented by farmers and government (George et al. 1999). To achieve a land use system with a water use more in balance with the pre-clearing scenario requires a significant enhancement of transpiration via the introduction of deep-rooted perennial species, and/or a significant enhancement of groundwater discharge via engineering options such as drainage and pumping. There is ongoing debate over how much area needs to be replanted in order to restore the original water balance, or even if such restoration is appropriate. Figure 3.1 is a conceptual attempt to define where different perennial systems could be deployed in the landscape to manage salinity. Where annual rainfall exceeds 600 mm and lateral flow within the soil profile is reliable, it is possible to consider plantation forestry and hillslope belts that intercept and use lateral throughflow. As rainfall becomes more limited, alternate methods of deploying woody perennials are required. Two systems have been proposed: woodlot rotations and alley plantings. Both systems are described in detail in this chapter, with an emphasis on the key scientific questions that face land managers in the development and implementation of revegetation strategies. High-watertable plantings are discussed elsewhere in this book. 37

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Figure 3.1: A hypothetical catchment depicting steep wetter uplands at the extreme left through to flat plains on the right. The bars above the diagram show the rainfall gradient and the zones where movement to trees is important. The bars below the diagram show the zones in the catchment suited to five different agroforestry strategies (used with permission, Stirzaker and Vertessy 2002).

How trees use water A hydrophysiological overview

Water flow through plants is generally considered to be a passive process in all but the tallest trees. Flow is driven by differences in water pressure. Surface tension at the evaporating leaf surface lowers the water pressure on the leaf surface and causes water to flow up from the roots. Appearing among the epidermal cells of the leaf are stomatal pores that open to the leaf interior. The size of the stomatal opening directly affects the rate of diffusion of water vapour and carbon dioxide across the leaf surface. Guard cells that respond to prevailing environmental conditions (light intensity, temperature, wind, leaf water potential) can control the size and shape of the stomatal opening and thus regulate the rate of water vapour and carbon dioxide exchange (Gates 1980). When the stomata are closed the only source of transpiration is by diffusion through the leaf cuticle. This accounts for approximately 10% of the total transpiration. When soils are dry, the root system is unable to extract water fast enough to keep up with evapora-

tive demand so the stomata usually remain closed or only partially open in order to regulate evaporative loss. Interestingly, some tree species, such as Eucalyptus grandis, also close their stomata in response to dry air conditions. This can occur even if they are irrigated and their roots have more than adequate water supplies. In the soil–plant–atmosphere continuum concept, the flow of water through the plant is often described using an analogue to Ohm’s law in electricity. Water is viewed as analogous to an electrical current and hydraulic conductance as analogous to electrical conductance. Resistances to flow are encountered along the water flow pathway from the soil to the evaporating surface (Figure 3.2). The first resistance is at the soil–root interface where there is a gradient in chemical-energy potential from the soil into the roots. As the soil dries, the rate of water uptake by the roots can become soilcontrolled – it becomes increasingly related to the unsaturated hydraulic conductivity of the soil, which becomes progressively less as the soil dries. Water then encounters a resistance within the roots as it moves through the fine root system towards the xylem vessels. Water in the xylem vessels

3 – Using trees to manage local and regional water balances

Figure 3.2: Simple Ohm’s law analogy of conductance in series. The total conductance (K plant) results from the conductance of the root, stem and leaves. Water flow is driven by the difference in water potential, which is lower at the leaf surface than at the root–soil interface.

responds to water potential and therefore moves up the stems into the leaves. Water must pass through mesophyll cells into the stomatal cavities whereby it is evaporated (either in the stomata or on the leaf surface) into the atmosphere. Importantly, this analogy to Ohm’s law leads to two significant predictions. First, the driving force of sap ascent is a continuous decrease in internal plant water pressure in the direction of sap flow. Second, the evaporative flux density from the leaves is proportional to the negative pressure gradient at any given cross-section along the transpiration stream. At any given point of a root, stem, leaf or vein we have (Tyree 1999): - dPx /dx = AE /K h + rgdh/dx

(Eqn 1)

where A is the leaf area supplied by a stem segment with hydraulic conductivity, Kh, and rg dh/dx is the gravitational potential gradient, where r is the density of water, g is acceleration due to gravity, h is the vertical distance and x is the actual distance travelled by water in the stream segment.

order to make informed decisions when selecting woody perennials for salinity control. The concept of ‘wilting point’ implies that when the leaf water potential, yl, falls during drought a ‘turgor loss point’ is reached (turgor pressure falls to zero) and the leaf loses its shape, or ‘wilts’. If cavitation (air entry) occurs in the xylem vessels then death usually results. Many perennial species maintain leaf shape through rigid leaf fibre cells, so wilting point is equated to the turgor loss point. Many native plants growing in regions with seasonal rainfall patterns appear to be drought evaders, while others are drought tolerators. One evasion strategy is for a plant to be deciduous, another is to have deep roots and a highly conductive hydraulic system. On the other hand, drought tolerators can withstand highly negative internal pressures by having a vascular system resistant to cavitation that would otherwise block the passage of water. Such species can be shallow-rooted or grow in saline environments. Responses to waterlogging

Plants can also experience dehydration if soils become saturated and oxygen diffusion to roots is too low to support metabolic activity, thereby diminishing their ability to conduct water (Kramer and Bouyer 1995). Feng et al. (2003) showed that at water contents around 0.3 cm3/cm–3 in sands and sandy loams and 0.34 cm3/cm–3 in non-swelling clays, the rate of oxygen diffusion drops considerably, to below a threshold value of 0.2 µg/cm–2/ min–1. At this threshold, many agricultural plant species fail to grow (Lety and Stolzy 1967). This is an important consideration in the Australian agricultural landscape, where salinity and waterlogging often occur in the same landscape locations. Waterlogging-tolerant plants usually have morphological adaptations in the roots that involve root thickening with an increase in porosity, thereby increasing the rate of oxygen diffusion to root tips. Modelling the root water uptake response

Responses to drought

Under drought conditions trees clearly use less water than when it is readily available; this affects productivity and the potential survival of perennial species. Land managers therefore need to understand how species respond to drought in

Modelling water movement between soil and plant roots requires some quantitative description of root water uptake from the soil. Direct measurement of this uptake is extremely difficult and it is commonly incorporated into a sink term in the governing flow equation (Simunek et al. 1996).

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The sink term usually varies as a function of the distribution of root density with depth (Landsberg 1989). A macroscopic sink term, depending only on soil water pressure head, was defined by Feddes et al. (1978): S = a]hgS max

(Eqn 2)

where Smax is the non-limiting water uptake and a(h) is a dimensionless function of soil water pressure head. Conceptually, the Feddes et al. (1978) model captures the water stress response of plants over the entire water potential range for saturation to wilting point (Figure 3.3). The response function in Figure 3.3 is characterised by five points that can be plant-specific. These points include:

s s s s

the soil water potential at which plant roots first begin to function (h1); the points between which soil moisture is nonlimiting and transpiration is controlled by atmospheric demand (h2 and h3); a segment after h3 where root water uptake diminishes rapidly to h3L; a more gradual decline in root water uptake to the point where root water uptake ceases due to insufficient soil supply (h4).

In practice, h3L is often ignored due to insufficient data and a straight line from h3 to h4 is assumed (as shown in Figure 3.3). The Feddes model has been applied widely to agricultural crops and soils, including barley (Al-Khafaf et al. 1989) and potatoes (Belmans et al. 1982), but it has not been applied specifically to Australian native species. Although other models of root water uptake exist (e.g. van Genuchten 1987), they do not depict the decrease in root water uptake due to oxygen limitations in the near-saturated range (between h1 and h2). Combined effects of waterlogging and salinity on plant water use

Most salt stress and water stress studies have been carried out separately and a large amount of data are available for only one of these stresses. Generally, the so-called multiplicativity concept is applied to the joint water and salt stress. This is based upon the product of the separate reduction terms for soil water osmotic heads (ho) and the soil

Figure 3.3: Transpiration sink term variable, a, versus soil water pressure head, h, for the Feddes model. Root uptake commences at h1, uptake is non-limiting between h2 and h3 and ceases at h4.

water pressure heads (h), as introduced by van Genuchten (1987). Homaee et al. (2002) present and discuss three categories of reduction function: additive, multiplicative and the conceptual combined method. After encountering limitations with both the additive and multiplicative reduction functions, Homaee (1999) combined the reduction functions of Feddes et al. (1978) with that of Maas and Hoffman (1977) and proposed a combined reduction function: a^ h, h 0 h =

h - ^h 4 - h 0h a x : 1 - 360 _ h *0 - h 0 iD h 3 - ^h 4 - h 0h (Eqn 3)

where h is the soil water pressure head, h0 is the osmotic head, h *0 is the osmotic threshold, h3 is the soil water pressure head threshold value, h4 is the soil water pressure head at wilting and the value 360 converts the salinity-based slope to osmotic head in cm (US Salinity Laboratory Staff 1954). This equation is valid for h o # h *o and ^ h 4 - h o h # h # h 3 . Each dS/m salinity beyond the threshold value (EC*) shifts the wilting point 360 cm to the left. This is a first approximation based on USDA Handbook 60 to transfer soil salinity to osmotic head. The model also differs conceptually from the additive and multiplicative models in that it assumes that the reduction function of Maas and Hoffman (1977) can be employed directly in the non-stress part of the Feddes et al. (1978) model.


fringe to reach a salt concentration after which the trees can no longer extract water from it is approximated by (Stirzaker et al. 1999): lq C t m = E c Cm - 1 m o

Figure 3.4: Illustrative effect of waterlogging and salinity on the sink term variable, a. The dotted line shows a reduction due to waterlogging in region I, and salinity reductions in regions II and III. The absolute amount of reduction (and the position of h4) depends on species and salinity concentration.

An example of the concept is shown in Figure 3.4. It is important that plant water uptake (and hence transpiration) reduces as a decreases. Therefore, salinity can magnify the reduction in plant water uptake due to waterlogging in region I (Figure 3.4), may reduce the non-limiting (in the absence of salinity) uptake in region II and reduce the uptake in region III as well as shift the wilting point to the left. Thorburn (1997) has shown that where watertable reductions have occurred, trees accumulate salt in the root zone which reduces their water uptake and therefore lessens their ability to sustain reductions in watertable levels. Salting out

In irrigated agriculture, the concept of a ‘leaching fraction’ is well understood: basically, a certain proportion of irrigated water must pass below the plant root zone to drains in order to prevent salt build-up in the root zone. Because roots exclude almost all the salt at the root surface during transpiration, the root zone salt concentration can increase through time if leaching is absent. Ultimately, a point may be reached where the plant can no longer extract water. This point may be confounded by waterlogging, but if we consider only the case of water use by a belt of trees from the capillary fringe above a watertable, then the time required for the capillary

(eqn 4)

where tm is the time in years to reach the maximum concentration, Cm (dS m–1), Co is the salt concentration in the groundwater (dS/m –1), E is the water use of the tree from the capillary fringe (mm/yr–1), l (mm) is the height of the capillary fringe above the the watertable where salt can be stored and q is the average water content of the capillary fringe (cm3/cm–3). Stirzaker et al. (1999) do not report on values of Cm for native vegetation. The actual levels of salinity that can be tolerated depend on tree species and soil factors, and are still poorly known. Morris and Thompson (1983) conservatively estimate a tolerance threshold to be in the range of 1.4–2.2 dS/m–1. George (1990) reported salinity levels up to 1.7 dS/m–1 in a growing plantation on a sandplain seep and Schofield et al. (1989) showed that watertables were lowered by eucalypts where the groundwater salinities were less than half seawater (2 dS/m–1). If we assume a Cm of 2. dS/m–1 then scenarios for typical soil and water use conditions in the 300– 600 mm rainfall zone of southern Australia suggest that plantations could be salted out before they reach 10 years of age unless there is some periodic flushing of the capillary fringe by above-average rainfall. Functionality of root architecture

Eucalypt roots have been reported to extend up to 20 m from the trunk of individual trees (Zohar 1985). At this extent, a regularly distributed density of 6 stems/ha would allow the outer edges of the root circles of adjacent trees to be contiguous if the trees are distributed in a regular pattern (Stirzaker et al. 1999). The implication is that a contiguous distribution of tree roots will effectively reduce groundwater recharge to near zero. However, the primary factor affecting the pattern of water extraction by plants from soils is the rooting depth. Many tree species have dimorphic root systems, with a relatively high density of roots in the topsoil and a sparser network of sinker roots (Dawson and Pate 1996). Although the root density is considerably higher in the topsoil, during the dry

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summer period in the Mediterranean climatic zone the topsoil can develop much lower water potentials than the leaves of the trees, yet tree water use is higher than during the winter period of low daily evaporation. The water use must therefore be supplied from deeper in the soil profile (Silberstein et al. 2002). The absolute depth to which tree roots extend depends on species, location and soil conditions. The roots of jarrah (Eucalyptus marginata) have been found as deep as 40 m (Dell et al. 1983) and longleaf pine to 10 m (Kramer, 1983). Recent work has shown that water extraction by replanted eucalyptus species can reach 6–8 m within three years (Sochacki et al. 2006). In native ecosystems, deeply rooted species may also provide water for shallower-rooted understorey species via hydraulic lift (Richards and Caldwell 1987). By this process, water is transported from deep roots (in contact with moist soil) to shallow roots overnight. Because the soil around the shallow roots has a greater potential than the deep roots, some rehydration of the soil occurs around the shallow roots. The use of this water by understorey species is parasitic (Dawson 1993) and a loss to the deep-rooted species. Having reviewed some of the physiological and environmental controls on water uptake by perennial vegetation, we now turn to defining the different groundwater systems that require recharge control. The definitions are largely taken from Hatton et al. (2002) and provide a general framework.

Groundwater flow systems Definitions of local, intermediate and regional scale systems

Local systems Local groundwater flow systems respond rapidly to increased groundwater recharge. Watertables rise rapidly and saline discharge typically occurs within 30–50 years of clearing of native vegetation for agricultural development. Bari and Smettem (2006a) show an 18 m rise in groundwater over 20 years in a partially cleared local scale catchment, followed by a significant increase in stream salinity (from fresh to saline) when the groundwater inter-

sected the stream invert and changed the character of the annual stream flow from intermittent to perennial (Bari and Smettem 2006b). Local systems can also respond relatively rapidly to salinity management practices, and afford opportunities to mitigate salinity at a farm scale. Typically, local systems have a horizontal scale of 1–3 km. Further subdivision into high and low discharge capacity local systems is possible. High discharge capacity systems are typically characterised by hillside seeps above relatively impermeable bedrock or local groundwater systems flowing through highly permeable colluvial sediments. Mostly, such systems are relatively fresh and highly transmissive and respond to revegetation. Low discharge capacity systems are more problematic to treat. They are generally confined and characterised by some form of impedance to flow such as a bedrock high, change of slope, change to lower hydraulic conductivity material or presence of a dyke, to name but a few. Salinity is usually high in such systems; salt stores in the regolith can also be high and become mobilised as groundwater rises. Intermediate systems Intermediate groundwater flow systems have a greater storage capacity and generally higher permeability than local systems. Horizontal scales of intermediate systems are of the order 5–10 km and are usually associated with alluvial (or lacustrine) fill in foothills and valleys. They take longer to ‘fill’ following increased recharge. Increased discharge typically occurs within 50–100 years of clearing of native vegetation for agriculture. The extent and responsiveness of these groundwater systems present much greater challenges for dryland salinity management than do local groundwater flow systems. Hatton et al. (2002) recognise three forms of intermediate systems:

s

typical broad valley systems characterised by discharge into low-lying areas as a result of reductions in transmissivity. Such systems are widespread in Western Australia and South Australia but in New South Wales are restricted to the Permian-recent sediments such as Yarraman Creek and Lake Goran. The Lockyer valley in Queensland is this type (Hatton et al. 2002);


s

discharge from confined or semi-confined aquifers at low points in the landscape, generally as a result of downstream reductions in transmissivity. This type of system is common in paleodrainage lines;systems associated with a linear zone of high transmissivity that cuts across a number of surface catchments and is recharged primarily by local recharge.

Regional systems Regional groundwater flow systems have a large storage capacity and permeability. Horizontal scales exceed 50 km and they take much longer to develop increased groundwater discharge than local or intermediate flow systems – probably more than 100 years after clearing the native vegetation. The full extent of change may take thousands of years. The scale of regional systems is such that farm-based catchment management options are ineffective in re-establishing an acceptable water balance. These systems will require widespread community action and major land use change to secure improvements to water balance. In Western Australia, the North Sterling basin has been identified as a regional system, with groundwater discharge occurring from the deeper aquifer into superficial aquifers that evaporate at the soil surface (Hatton et al. 2002). In eastern Australia, the catchments of the Riverine Plains are of this type, as are the Cooke Plains in South Australia. Transient shallow (perched) systems Ephemeral ‘perched’ aquifers are can occur above a major aquifer system at the interface between materials of contrasting texture and at a hillslope scale. They are typically shallow, small in extent and often fresh (George et al. 1997). Typically, perched systems can be found in association with duplex (texture contrast) soils (George and Conacher 1993; White et al. 2000), or where surface soils have well-developed macrostructure that permits preferential flow to occur at the soil–bedrock interface. Because perched groundwater flow (throughflow) is generally fresh, in some locations it can provide a major source of water (but not salt) to saline seeps (George and Conacher 1993) and provide dilution flows to rivers. Perched watertables can also provide rapid recharge to underlying

permanent groundwater systems via preferential flow (Williamson 1973; Dunin 2002). Interactions between trees and runoff

In some catchments, understanding the interactions between trees and runoff is critical for environmentally sustainable management. This is particularly so if maintenance of freshwater flows is required for ecological functioning in rivers, or if safe groundwater abstraction rates are to be determined. Physically based catchment hydrologic models can provide insights into the potential effects of proposed replanting strategies within catchments, but need to be coupled with detailed site appraisal for planning salinity management strategies. The details of such models are beyond the scope of this chapter; different types of hydrologic models used for this purpose have been compared by Beverly et al. (2005).

Ecological optimality and the groundwater balance The concept of ecological optimality (Eagleson and Tellers 1982) suggests that the canopy size of natural vegetation adjusts to maximise relative mean soil water concentration. Specht (1979) and Specht and Specht (1989) present empirical evidence supporting this concept for Australian conditions. From a synoptic perspective, the hydrologic implication is that in the 30°C) during flower development in cereals may severely affect yield and quality. Sensitivity to temperature occurs at anthesis and early grain development and can reduce grain number and single-grain weight. The negative impact of hot dry days on wheat yield was measured in a semi-arid region of Israel. The general response to the occurrence of hot windy days was a yield decrease of approximately 1.2 t/ha –1 in low-rainfall conditions and 0.6 t/ha–1 in higherrainfall conditions, compared with an average yield of 2.9 t/ha–1 (Lomas and Shashoua 1974). The sensitivity of field crops to hot dry wind events at flowering is a potentially significant factor affecting final yields. The severity of this effect is not clearly understood due to a lack of knowledge of the processes involved and plant responses to the environmental drivers (temperature, humidity, wind speed, soil moisture and duration) at the time of an event. Lodging Two main types of lodging may result from severe wind (Cleugh et al. 1998). Stem lodging occurs when the plant stem breaks or bends, while root lodging occurs when the entire root system fails and the plant falls over. Cereal breeding programs have developed cultivars that are relatively resilient to stem lodging, while the wet conditions conducive to root lodging are not prevalent in the cereal-

growing regions of Australia. For these reasons, lodging as a form of direct wind damage to crops is not perceived as a large problem. Furthermore, even if a lodging event occurs, most farmers believe that the grain can still be harvested and so yield losses are minimised. Another form of lodging of concern in southern Australia is the damage and removal of barley heads in response to hot dry winds at maturity. This was recognised in the 1960s in South Australia, where the Australian Bureau of Meteorology conducted a series of field experiments to develop predictive relationships between head losses and meteorological conditions, to provide an early warning forecasting service for the barleygrowing regions of South Australia. While a range of damage is possible, from fractured necks to damaged spears, the greatest yield losses result when the fracture of the neck leads to loss of the seed head. Yield losses from this type of damage can be minimised by rolling or windrowing the crop if sufficient advance warning of adverse weather is provided.

Australian National Windbreaks Program There has been almost a century of international windbreak research into the diversity of mechanisms by which shelter can enhance plant growth and we know that the intensity of their effects is determined by the climate, soil and farming system. Australia has a unique mix of these, thus international research can only say so much about the likely significance of windbreaks in Australian farming systems. In Australia, there was very little windbreak research before the National Windbreaks Program (NWP), but what existed created an atmosphere of encouragement for propagating windbreaks across our dryland farming systems (Bicknell 1991 and Burke 1991, described in Nuberg 1998). However, natural resource management agencies need scientifically rigorous quantitative information before they can convincingly promote windbreaks. The NWP provided this quantitative understanding of the interaction between windbreaks, microclimate and crop and pasture growth, not without some surprises. It provided some definitive answers to

5 – Trees protecting dryland crops and soil

BOX 5.3

Shelter effects on microclimate and plant growth – results of NWP 1993–98 Bleed flow/competition zone (0–2 H) s

Competition for water, light and nutrients reduced yields at all field sites.

s

Windbreak structure is important as gaps can lead to wind erosion and sandblasting damage.

s

Shading can offset increases in air temperature that result from shelter (Figure 5.2).

Figure 5.2: Shading can offset increases in air temperature.

Quiet zone (2–8 H) s

Calmer, warmer and/or more humid by day.

s

Reduced soil evaporation may improve crop establishment.

s

Atmospheric demand can be increased or decreased, depending on the humidity of the regional flow. In dry conditions, a reduction in atmospheric demand may lead to improved water use efficiency. This translates to more biomass and/or yield for the same water use as the crop upwind, or less water use than the upwind crop for the same biomass and/or yield.

s

While enhanced phenological development and biomass production are possible, this does not always translate into yield gains.

Wake zone (>8 H) s

Effects of wind shelter on temperature and humidity are small.

s

Shelter from wind reduces the risk of direct damage to plants from leaf tearing and stripping, plant lodging and sandblasting.

the values of windbreak in Australian dryland farming systems. The partners involved in the NWP were spread broadly in terms of geography and types of farming systems studied. In Western Australia, research-

ers at Esperance studied wheat, canola, barley and lupins; in South Australia at Roseworthy wheat, canola and faba beans were studied. In Victoria, wheat and lupins were studied at Rutherglen (north Victoria) while grazed perennial pasture was the

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Figure 5.3: Relative wind speeds for model windbreaks of three different porosities (Cleugh 2003). b is the windbreak porosity.

focus at Hamilton. In Queensland, maize and potatoes were studied at Atherton, and at Warwick the crop modellers gathered extra data from studying mung beans and irrigated wheat. The research was largely in the form of field experiments, using both the conventional quadrant-plot measurements and GPS-assisted yield monitoring, complemented by experiments using artificial shelters, wind tunnels and crop simulation models. Aerodynamic and microclimate changes behind windbreaks

A summary of the effects of shelter provided by windbreaks on microclimate and plant growth is given in Box 5.3. These were determined using wind tunnel and field experiments (Cleugh and Hughes 2002). The summary shows the presence of two zones downwind of a windbreak. The quiet zone is the zone of maximum wind speed reduction, where it is possible to detect significant modification of microclimate variables such as increased daytime temperature, humidity and reduced soil evaporation. The speed of the wind in the quiet zone has been reduced by transferring its kinetic energy into tree movement as it passes through the windbreak. The wind that passes over the top of the windbreak forms a turbulent mixing layer which eventually mixes with air at the ground surface in the wake zone. The wind speed is still reduced in this wake zone, but the microclimate effects are much smaller. Before the NWP, porous windbreaks were often considered more useful than less-porous windbreaks because they absorb more energy from the wind. While a denser windbreak may reduce wind

speed more (to a maximum of about 80%), the effect was believed to extend to a reduced distance downwind because dense windbreaks create a faster-growing mixing layer that has the effect of more quickly re-establishing upwind conditions. Consequently, the windbreak literature often states that a windbreak with 40–50% permeability gives the greatest area of shelter. However, an important conclusion from the NWP is that for the range of porosities of windbreaks likely to be found in Australia (30–70%), there is little difference in the amount of sheltered area behind windbreaks (Figure 5.3). For practical purposes, the downwind extent of shelter behind a windbreak is determined by windbreak height, wind direction and surrounding terrain. Nevertheless, porosity does determine wind speed and the degree of shelter is roughly similar to windbreak density, i.e. a windbreak with 30% porosity will reduce wind speed to 70% of the open-field speed at the most sheltered location. Consistency of shelter is another factor affecting the practical value of windbreaks. The synoptic regime affecting cropping systems of southern Australia is characterised by a pattern of alternating high- and low-pressure weather systems. The high-pressure system brings fine weather and light easterly winds, followed by north-westerly winds ahead of the cold front associated with lowpressure systems over the Southern Ocean. These winds may strengthen and grow colder as they swing around to the west and south-west. Many farm windbreaks in southern Australia are oriented in a north-south direction to protect crops and livestock from these westerly winds. Long windbreaks still provide significant shelter, though over a shorter distance, even when the wind flows obliquely within 45° of normal. However, the microclimate advantages for plant growth in the quiet zone will be restricted to times when the wind is coming from the west. At other times these advantages are lost and the aggregate microclimate advantage over the whole growing season is diminished. The microclimate advantages of shelter concern reduced turbulent flux, or mixing and movement, of heat and water between the soil, crop and atmosphere and therefore more conservative use of soil water and more efficient photosynthesis. All this


happens in the relatively small area of the quiet zone. As soon as the wake zone is reached the turbulent fluxes are actually greater than upwind of the windbreak. In the NWP, the little evidence there was for increased soil water storage in the quiet zone showed that it resulted in only slightly greater crop biomass, not greater grain yield (Nuberg and Mylius 2002). This is not surprising given the complexity of the effect of shelter on transpiration outlined above (referring to Equation 1). Combined with the inconsistent shelter due to weather patterns, microclimate advantages are subtle and limited. Indeed, crop simulations using APSIM (Agricultural Production Systems Simulator) estimated that wheat grown in the shelter of a hypothetical windbreak at Roseworthy, South Australia, which only experienced winds originating within a 90° arc perpendicular to the windbreak, would show a yield gain of 7.2% at 5 H (Carberry et al. 2002). As shown below, such yield enhancements were not realised because of the inconsistency of shelter, as well as soil factors, under field conditions. Crop yield responses to shelter

The field responses observed in the NWP were as varied as the farming systems studied, but the results showed that in general two broad areas of plant response can be expected downwind of a windbreak: a zone of reduced yield caused by competition from the trees that extended from 1 H to 3 H, and a zone of unchanged or slightly increased yield that extended downwind from there to 10–20 H. The effect of shelter-induced microclimate changes on crop yields was not very great compared with measurements made overseas. This was particularly the case for cereal crops, unsurprising given that they are naturally well-adapted for windy grassland environments. Simulation models using long-term historical data indicate that this relatively modest response to shelterinduced microclimate changes will be found in other crop-growing regions in Australia (Carberry et al. 2002). Nevertheless, a closer look at some specific yield observations in the NWP reveals that there may still be very good reason for planting tree windbreaks in Australian farming systems. On the south coast of Western Australia the yields of various crops were measured in the shel-

Figure 5.4: Mean crop yield in the lee of windbreaks relative to open conditions (20–30 H) in the Esperance district, WA, in a dry year (▲ 1994), years receiving average rainfall (● 1995–97) and a year with severe wind erosion (■ 1996) (Sudmeyer et al. 2002). Distances from windbreak are expressed as multiples of tree height (H).

tered zone of windbreaks at 63 fields over three years (see Figure 5.4). The strength of the response varied with crop type, windbreak orientation and seasonal condition. Windbreaks had the strongest shelter effect on cereal yields in dry years. However, even then the net result was minimal because trees compete for resources as far as 4 H into the paddock. Lupins were the only crop that consistently showed higher net yields on a sheltered paddock, with a mean increase from 1–20 H of 4% (Sudmeyer et al. 2002). As part of the follow-up study on severe wind effects, grain yield data were collected from windbreak sites through the eastern agricultural districts of South Australia using a harvester equipped with a yield monitor. Variability in shelter response occurs within field crops for a variety of reasons such as soil factors, nutrients, disease, rainfall distribution and prior land use in addition to the impact of wind. Analysis is a difficult task, particularly with the limited number of data points that can be gathered in quadrants harvested by hand or plot harvester. The use of a combine harvester fitted with precision yield-monitoring equipment to map spatial variation in grain yield gave an opportunity for analysis of variability that had previously been unavailable in field crops. Figure 5.5 illustrates the type of generalised yield map generated using this approach. It shows a yield map for faba beans, the crop most responsive to the shelter provided by windbreaks. This is

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Figure 5.5: Yield map of faba bean in the lee of a NE–SW windbreak providing shelter from north-west winds in South Australia. The map clearly illustrates decreased yield due to competition adjacent to the windbreak, followed by a zone of significantly increased yield in the sheltered zone (Bennell and Cleugh 2002).

because faba beans are very sensitive to microclimate as well as flower abortion under hot dry winds. Cereal crops, with leaves and flowers adapted to windy environments, are much less responsive. Figure 5.6 illustrates the aggregate results from the South Australian yield monitor trials. While cereal crops showed a modest yield enhancement in the sheltered zone, this is virtually negated by the competition from the windbreak trees and confirms the earlier results from the NWP. In contrast, significant yield returns could be consistently expected from faba beans.

The study concluded that in areas where cereals are the predominant crop, the net yield gains due to shelter from windbreaks would be small, even if root pruning could be successfully undertaken. Much better gains are expected from legume crops such as lupin and faba bean, which seem much more sensitive to shelter. The relatively poor net yield enhancements in cereals due to shelter in Australia are due to the relative wind-hardiness of cereals and the predominance of shallow soils with low water-holding capacity. The effect of shelter on crop energy balance and water use is complex and may increase or decrease transpiration and the conservation of soil moisture. It appears that, in Australian agricultural systems, where favourable microclimatic conditions may be observed in the sheltered zone, these do not usually lead to large yield benefits. They certainly do not increase net paddock yields, when tree–crop competition is included. The largest yield gains observed in the NWP came from protection against infrequent strong winds that cause plant damage and soil erosion. We now turn to the role of shelter in protection against physical damage.

Protecting crops from physical damage A key finding from the National Windbreaks Program is that wind damage resulting from severe wind events is likely to play a much more important role than previously expected in the response of crop yields to wind shelter. A cost–benefit analysis of windbreaks in Western Australia showed them to be a good form of insurance if sandblasting events occur more than once every seven cropping seasons

Figure 5.6: Enhanced yields and net gain in the sheltered zone of three crop types between 1997 and 2000 in South Australia (Bennell and Cleugh 2002).


(Jones and Sudmeyer 2002). Following the first phase of the NWP, a follow-up study investigated the extent to which protection from damaging winds can modify productivity. This involved wind tunnel experiments and economic simulation models (Bennell and Cleugh 2002; Bennell et al. 2007). Wind tunnel experiments Wind damage events occur intermittently; they are characterised as low frequency–high magnitude events and are analysed in terms of probabilities of occurrence. The main risk factors for mechanical damage in southern Australia include sandblasting, flower abortion and head loss in mature barley crops. Direct physical crop damage by leaf tearing is not considered a problem for field crops. As the occurrence of damaging wind events is unpredictable, the best way to quantify the relationships between wind damage and crop yield is to use a portable wind tunnel. Such devices can provide the high wind speeds needed to simulate sandblasting of seedlings, a hot dry wind on flowering plants or the impact of wind only on yield. Wind tunnel experiments found that the grain yield of narrow-leaf lupins was reduced by 18% for a modest sandblasting event (a sand flux of 248 kg/m) that has the potential to occur every year. Extreme events are known to be characterised by sand fluxes over an order of magnitude greater. Where the wind tunnel was used to simulate hot dry wind events with an air temperature of 30°C and low relative humidity, rates of flower abortion increased with wind speed up to 12 m/s, by which point there was a 35% reduction in pod set below that observed in the control plants at flower development stages up to and including anthesis. As the flowers develop they become less sensitive to the hot windy conditions, with abortion declining gradually from early developmental stages and showing a sharp decline in sensitivity after anthesis. Once the pod has set it is not vulnerable to extreme wind conditions and has a high chance of continuing to develop if moisture is available. The actual yield reduction observed in the field will depend on seasonal conditions and the timing of the event. Late in the growing season, lack of soil moisture will stop plant growth so flowers affected by the wind event may not reach a harvestable stage.

Examination of long-term climate records can provide insight into the risk of damage in a particular region. For example, meteorological records for Adelaide were used to estimate the probability of damaging events affecting pod set and grain yield in pulse crops in South Australia. Events where temperatures exceeding 28°C coincided with maximum wind speeds in excess of 10 m/s occurred with a frequency of one severe event every 7.2 years in September and one every 2.4 years in October. The September events are most likely to have an impact on yield, as flowers setting during this period have the greatest chance of developing through to harvestable pods.

Cost–benefit analysis of windbreaks The NWP provided great insight into the mechanisms of shelter as they apply in Australian farming systems. There remained the question of the net financial costs and benefits of windbreaks, given that there are costs associated with planting and maintaining them and that their benefits may not be realised every year. The analysis required is complex because of the numerous interactions between physical factors (climate, soil types, windbreak design), biological factors (tree and crop species), management (rotations, tillage, tree-root ripping) and the financial costs of windbreak establishment and potential for commercial products. Bennell and Cleugh (2002) developed a simulation model to determine the potential net financial value of windbreaks integrated into cropping systems of South Australia. The model simulated a hypothetical South Australian farm with an array of possible regional climates, crop rotations, soil types, tillage practices, windbreak type and spacing and windbreak management (root ripping to reduce competition with crops). The model incorporated existing crop yield, sand flux and farm economic models and used 25 years of historical climate records to predict the net financial returns to the hypothetical farm business under different scenarios. The simulated farm comprised three separate fields with each crop of the rotation cycle being cultivated in each year. This approach captured the impact of infrequent severe damaging events on all the constituent crops.

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The model showed quite clearly that the net financial benefit of windbreaks varies greatly with region (soils, climate) and cropping system. For example, the outcomes for three scenarios representing current agricultural practices in South Australia were as follows.

s

s

s

Windbreaks will provide positive financial returns where there are lupins in the crop rotation and there is a significant risk of sandblasting, i.e. lower annual rainfall and erodible soils. While this risk could be managed by maintaining high cover levels, not all farmers practise conservation tillage. Other factors such as drought will also limit the amount of cover. In such situations, windbreaks will provide positive financial returns. Systems with multiple parallel belts (alley farming) will provide more shelter, and net financial return, than a single windbreak along a paddock boundary. In drought years, when maintaining above-ground cover is particularly difficult, windbreaks will be the only way to reduce the risk of wind erosion, yielding a much longer-term financial and environmental gain as wind erosion strips paddocks of valuable nutrient-rich topsoil. Where cereal crops are grown in rotation with a pasture rather than lupins, the financial returns from windbreaks are negative. Windbreaks would be best considered as a strategy to manage the risk of infrequent severe wind events causing soil erosion, sandblasting, flower abortion and head loss but may not be economically profitable, even with root ripping. In heavier soils with improved rainfall, faba beans are often part of the crop rotation. Simulations showed that significant yield gains in faba beans could be achieved through the use of windbreaks. However, the minimal yield gains in the cereal part of the rotation, combined with other windbreak costs (establishment, maintenance and lost productive land), negated gains in the faba beans and the outcome was a negative return on the windbreak investment. Rootripping windbreaks in these higher-rainfall areas led to only modest yield gains in the cereals and did not significantly improve the economic returns. Heavy soil types removed the impact of sandblasting on productivity, so mitigated the production benefits provided by windbreaks.

These results complement those from the south-east coast of Western Australia where, without wind damage, windbreaks reduced net agricultural returns (Jones and Sudmeyer 2002). In combined grazing-cropping systems, windbreaks spaced at about 200 m can improve net returns through the reduction in stock losses conferred by shelter (Bennell and Cleugh 2002). In the cropping and grazing lands of Australia, the multiple benefits of windbreaks must be captured. Much of the benefit will arise from the reduced risk of infrequent but damaging events, in both financial and environmental terms.

Conclusion Windbreaks provide multiple services to cropping systems of southern Australia. While the crop itself retains its primacy in a farming system, windbreaks can be established as part of a whole-farm plan to confer benefits of:

s s s s s

management of groundwater (Chapter 3, this book) and surface water and pollutants (Chapter 4); connecting remnant vegetation patches and providing habitat (Chapter 6); aesthetic plantings to soften and nourish the visual landscape (Chapter 8); opportunities for low-rainfall timber (Chapter 10) or firewood production (Chapter 11); enhancing the productivity of the livestock component in mixed farming systems (Chapter13).

This chapter has shown that the value of windbreaks in enhancing the productivity of cropping systems lies mainly in protection from mechanical damage rather than from microclimate effects. This is because our cropping systems are dominated by cereal crops which are naturally adapted to windy environments. Other crop types, such as pulses and oilseeds which are open-flowering and have above-ground growth meristems, respond better to shelter (Grace 1988). These other crops also respond favourably to microclimate effects of shelter, especially in dry years. However, the combined loss of production due to competition effects at the tree–crop interface, and the loss of land from the windbreak itself, are likely to negate the beneficial effects of shelter in many southern Australian cropping systems. Furthermore, a fence-line wind-


break will shelter only part of a cropping paddock, and as wind will come from all directions over the course of a season there is little case for expecting a net financial gain from establishing windbreaks on the basis of enhanced crop yield alone. There are clear exceptions to this. Windbreaks will provide positive financial returns where there is a significant risk of sandblasting and where windsensitive plants play a significant role in the crop rotation, e.g. the southern region of Western Australia where lupins are commonly grown. Sandblasting is the most likely negative wind effect in areas of light soils and frequent severe wind events. Sandblasting can also be managed by retaining high levels of ground cover, which is perhaps a better solution than windbreaks as minimum tillage options have other benefits for the farm. However, not all farmers practise minimum tillage and drought will limit the amount of cover. In drought years, when maintaining cover is particularly difficult, windbreaks will be the only way to reduce the risk of wind erosion, yielding a much longer-term economic and environmental gain as wind erosion strips paddocks of valuable nutrient-rich topsoil. For cropping farmers, windbreaks may be a good insurance policy against crop damage and soil erosion, especially in dry years. As our climate is changing, the incidence of hot dry years and strong winds may increase and the protective value of windbreaks will grow. It would be folly to dismiss the value of windbreaks based on financial evaluations from historical climate data alone. Evaluation must include the other functions of the trees. Given that there is a role for windbreaks in cropping systems, what should be their design? The principles outlined in Box 5.1 are universal and can be applied to the design of windbreaks for Australian cropping systems. We know that:

s s s

the consistency of windbreak porosity (lack of gaps) is probably more important than the porosity itself; within the range of windbreak porosities found in the field, the actual location of maximum shelter does not change; the relationship between porosity and level of wind speed reduction is relatively linear.

The vexing question for farmers and land use planners is how much of the landscape can be usefully occupied by windbreaks.

Fence-line windbreaks offer limited shelter to large cropping paddocks. For example, the actual sheltered zone of a 5 m high windbreak will only extend a maximum of 95 m into the paddock. A 40 ha paddock may be over 600 m so multiple windbreaks or alley farming systems will be needed to spread the shelter across the paddock. A practical minimum spacing for such a system would be intervals of 20–25 H, which provides useful wind speed reductions and thereby protection from mechanical damage. Livestock also greatly benefit from such shelter. Depending on orientation, this density of planting means that the field will be sheltered from winds from most directions. Closer spacings at intervals of about 10–12 H would secure the maximum amount of shelter by increasing the frequency of quiet zones. Such a system may only be worthwhile when the trees themselves provide some productive value to offset the opportunity cost of lost cropping, and the interbelt crop would benefit from enhanced microclimate. This may be the case for a fodder-shrub alley system with lucerne. However, most foddershrubs rely on clear management of grazing duration to maintain their productive potential. They may be best in blocks in areas of the farm where lower returns from cropping could be expected. Tree belt or alley systems are suggested for the production of oil mallee in Western Australia (Chapter 15) because they trap excess water in the landscape via surface and subsurface lateral flows. In this situation, the shelter for the interbelt crop is a benefit secondary to those of groundwater management and salinity mitigation. This is really the core guiding rule of effective windbreak design in cropping systems – that they provide multiple functions that enhance the wholefarm productivity and sustainability.

References Bennell M and Cleugh H (2002) ‘Integrating windbreaks into southern Australia farming systems: managing the impacts of severe wind events’. Report for the RIRDC/Land and Water Australia/FWPRDC/MDBC Joint Venture Agroforestry Program, RIRDC Project Nos SAR-19A and CSL-10A, Canberra. Bennell MR, Leys JF and Cleugh HA (2007) Sandblasting damage of narrow-leaf lupin (Lupinus

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angustifolius L.): a field wind tunnel simulation. Australian Journal of Soil Research 45, 1–10. Bicknell D (1991) The role of trees in providing shelter and controlling erosion in the dry temperate and semi-arid southern agricultural areas of Western Australia. In Proceedings of a National Conference. The Role of Trees in Sustainable Agriculture. Albury, Vic., pp. 21–39. National Agroforestry Working Group/Bureau of Rural Resources, Dept of Primary Industries and Energy, Canberra. Bird PR (1998) Tree windbreaks and benefits to pastures in temperate grazing systems. Agroforestry Systems 41, 35–54. Brenner AJ (1996) Microclimate modifications in agroforestry. In Tree–Crop Interactions: A Physiological Approach. (Eds CK Ong and P Huxley). CAB International/ICRAF. Burke S (1991) Effect of shelterbelts on crop yields at Rutherglen, Victoria. In Proceedings of a National Conference. The Role of Trees in Sustainable Agriculture. Albury, Vic., pp. 89–99. National Agroforestry Working Group/Bureau of Rural Resources, Dept of Primary Industries and Energy, Canberra. Butler P, Davies WJ and Leys JF (1995) ‘Dust storms in South Australia on 24–25th May 1994’. Dept of Primary Industries, South Australia. Technical Report no. 243. Carberry PS, Meinke H, Poulton PL, Hargreaves JNG, Snell AJ and Sudmeyer RA (2002) Modelling crop growth and yield under the environmental changes induced by windbreaks. 2. Simulation of potential benefits at selected sites in Australia. Australian Journal of Experimental Agriculture 42(6), 887–900. Cleugh HA (2003) ‘Trees for shelter: a guide to using windbreaks on Australian Farms’. Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. 02/059. Cleugh HA and Hughes DE (2002) Impact of shelter on crop microclimates: a synthesis of results from wind tunnel and field experiments. Australian Journal of Experimental Agriculture 42 6), 679–701. Cleugh HA, Miller JM and Bohm M (1998) Direct mechanical effects of wind on crops. Agroforestry Systems 41, 85–112.

Cleugh HA, Prinsley R, Bird PR, Brooks SJ, Carberry PS, Crawford MC, Jackson TT, Meinke H, Mylius SJ, Nuberg IK, Sudmeyer RA and Wright AJ (2002) The Australian National Windbreaks Program: overview and summary of results. Australian Journal of Experimental Agriculture 42(6) 649– 664. Grace J (1988) Plant response to wind. Agriculture, Ecosystems and Environment 22/23, 71–88. Huxley PA, Pinney A, Akunda E and Muraya P (1994) A tree/crop interface orientation experiment with a Grevillea robusta hedgerow and maize. Agroforestry Systems 26(1), 23–45. Jones H and Sudmeyer RA (2002) Economic assessment of windbreaks on the southeast coast of Western Australia. Australian Journal of Experimental Agriculture 42(6), 751–761. Kelleher FM (1984) Climate and crop distribution. In Principles of Field Crop Production. (Ed. JE Pratley). Sydney University Press, Sydney. Leys JF and McTainsh GH (1994) Soil loss and nutrient decline by wind erosion: cause for concern. Australian Journal of Soil and Water Conservation 7(3), 30–40. Lomas J and Shashoua Y (1974) The dependence of wheat yields and grain weight in a semi-arid region of rainfall and on the number of hot, dry days. Israel Journal of Agricultural Research 23, 113–121. Miller JM, Bohm M and Cleugh HA (1995) ‘Direct mechanical effect of wind on selected crops: a review’. CSIRO Centre for Environmental Mechanics. Technical Report no. 67, 1–68. Nuberg IK(1998) Effect of shelter on temperate crops: a review to define research for Australian conditions. Agroforestry Systems 41, 3–34. Nuberg IK and Mylius SJ (2002) Effect of shelter on the yield and water use of wheat. Australian Journal of Experimental Agriculture 42(6), 773–780. Ong CK, Corlett JE, Singh RP and Black CR (1991) Above and below ground interactions in agroforestry systems. In Agroforestry: Principles and Practice. (Ed. PG Jarvis) pp. 45–58. Elsevier, Amsterdam. Pitcairn CER, Jefree CE and Grace J (1986) Influence of polishing and abrasion on the diffusive conductance of leaf surface of Festuca arundinacea Schreb. Plant Cell and Environment 9, 191–196.


Stoddard FL (1993) Limits to retention of fertilized flowers in faba beans (Vicia faba L.). Journal of Agronomy and Crop Science 171, 251–259. Sudmeyer RA, Adams MA, Eastham J, Scott PR, Hawkins W and Rowland IC (2002) Broadacre crop yield in the lee of windbreaks in the medium and low rainfall areas of south-western Australia. Australian Journal of Experimental Agriculture 42(6), 739–750.

Thornley JHM and Johnson IR (1990) Plant and Crop Modelling: A Mathematical Approach to Plant and Crop Physiology. Clarendon Press, Oxford. Wu YY and Dalmacio RV (1991) Energy balance, water use and wheat yield in a Paulowniawheat intercropped field. In Agroforestry Systems in China. (Eds ZH Zhu et al.) pp. 54–65. CAF/IFDC.

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6 Biodiversity and habitat enhancement David Salt and David Freudenberger

This chapter aims to define the concepts of biodiversity and habitat, and discuss the role that agroforestry can have in enhancing biodiversity in agricultural landscapes. A set of principles is outlined that will help in planning and managing an agroforestry enterprise for biodiversity. Case studies are provided to illustrate the concepts.

liability. Not only might it contain little of value for local native biota, but it could also have serious negative impacts on what is already present in the area. For example, if the plantation is established at the expense of remnant native vegetation it is unlikely that plantation will ever produce a net benefit for the native biodiversity of that area.

Introduction

Defining biodiversity

While there are no conclusive studies establishing the precise biodiversity benefits that agroforestry can provide, most of the evidence strongly suggests that tree plantations, even if they contain a mix of natives, are not a substitute for a native forest or a patch of remnant native vegetation. They simply do not have the structural complexity, the mix of ages nor the range of resources necessary to support a diverse assemblage of native animals and plants. However, an agroforestry planting can provide several resources necessary for some of the wildlife and native plants present in a region. Agroforestry, if planned sympathetically with the retention of existing native vegetation, can have significant conservation benefits. Further, it is possible to modify a plantation and its management in many ways that do not cost too much in lost production or forgone opportunities. An agroforestry planting may serve as an asset to biodiversity if it is appropriately established and maintained, but a poorly implemented plantation can prove the opposite – a damaging environmental

The National Strategy for the Conservation of Australia’s Biological Diversity (released in 1996) defined biodiversity as: The variety of life forms: the different plants, animals and micro-organisms; the genes they contain; and the ecosystems they form. It is usually considered at three levels: genetic diversity, species diversity and ecosystem diversity. This succinct and widely accepted definition does not make explicit the inherent and underpinning structural and functional aspects of biodiversity. Noss (1990) recognised that biodiversity is not simply the number of genes, species or ecosystems in a defined area. Knowing that one area contains 500 species and another contains 50 species does not indicate how these species are arranged (structured) or what they do (function). Noss developed a simple conceptual framework for identifying specific and measurable indicators of compositional, structural and functional biodiversity (Figure 6.1). Biodiversity is much more than a list of species 87


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Figure 6.1: A simple conceptual framework for identifying specific and measurable attributes of biodiversity (adapted from Noss 1990).

found in a stand of trees. Rather, it is how these species are arranged or structured in space and how they interact with their environment that matters. So, for the following discussion we use a broader definition of biodiversity: it is the variety of life, what it is called (composition), how it is arranged (structure) and what it does (function) at a range of scales, from genetic to global (based on Noss 1990). An example of composition, structure and function as distinct aspects of biodiversity can be seen in before-and-after surveys of forest plots in the Brindabella Mountains, to the west of the ACT, following the January 2003 bushfire (Doherty and Wright 2004) (Figures 6.2a, 6.2b). While the floristic composition of the forest is returning rapidly to pre-fire composition after an initial increase in species richness post-fire, the structure of the forest was greatly simplified to a few burnt stags with epicormic growth and a low understorey of regenerating plants, and this will take many years to re-establish a similar pre-fire structure. In the case of areas in the northern parts of the Brindabellas on acid volcanic soils, water quality was maintained after the fires. However, in areas on granitic geology in the Cotter catchment, the function of the forest was greatly altered by the fire. For six months after the fire, the forest in these areas no

(b)

Figure 6.2: (a) Before and (b) after photos of forests in the Brindabella Mountains, west of Canberra. The formerly forested landscape was converted into a mass of burnt spindles and resprouting shrubs. Floristically the ecosystem is the same but its structure and function have been dramatically modified (photos by Michael Doherty).

6 – Biodiversity and habitat enhancement

longer effectively filtered water runoff and the dams in the Cotter catchment became too full of suspended sediment to be used for Canberra’s water supply. This chapter will focus on how agroforestry can enhance the compositional and structural attributes of biodiversity. Chapters 2–4 have already discussed how biodiversity, particularly trees, can enhance the functionality of the landscape (e.g. water balance and nutrient cycling).

Enhancing habitat Habitat refers to the environment in which individuals and populations of a species live. Habitat is a continuum across a landscape, and can also be thought of as the environmental niche that a species occupies. There are two key features of habitat that must be kept in mind: irreplaceability and complementarity. Some habitats cannot be replaced. Old growth forests are unique. They are the result of millions of years of evolution in a given place and time. Once an old growth habitat, be it a wetland, grassland or woodland, has been cleared, it cannot be replaced in its entirety. The best we can do is mimic or replace some aspects of old growth habitat, but agroforestry can never fully replace the remarkable and ancient diversity of species and functions found in old growth forests. The irreplaceability of these forests has increased as they have become fragmented by clearing for agriculture, plantation forests and urbanisation. Any patch of habitat occurs in a context of other habitat patches across any given landscape. Habitat patches are complementary, that is, they contribute to a greater whole. An agroforestry planting will complement other habitats depending on what else is around. A 1 ha planting of blue gums has little complementary habitat value if it is just another hectare in a 10 000 ha blue gum plantation. However, the same 1 ha planting may have high complementary value if it buffers a remnant patch of old growth woodland otherwise surrounded by a sea of wheat stubble. When assessing the possible contribution an agroforestry planting (or design) might make to biodiversity, it ultimately comes back to these two key features: irreplaceability and complementa-

rity. That assessment can be simply phrased as two questions:

s s

Irreplaceability: does the planting involve the destruction or decline of existing habitat (such as remnant forest or native grassland)? Complementarity: does the planting offer additional resources that will complement or protect existing habitat?

Sometimes the answer is obvious. For example, if the establishment of a plantation involves the clearing of remnant vegetation then it will have a negative impact on an area’s biodiversity. No matter what is done with the planting, it won’t be able to fully replace the composition, structure and function of the remnant vegetation. For example, many of the features of a patch of mature native forest (e.g. tree hollows) often take more than 100 years to develop. Sometimes the outcomes are less clear and are difficult to assess in advance. If a planting is done on land previously dominated by exotic pasture and involves growing a mix of local native trees, it might well have a positive impact on an area’s biodiversity. The value of that impact, however, will depend on a range of factors relating to the location, configuration, composition, complexity and management of the planting. The following section presents a set of principles that will help maximise the benefit of a planting for biodiversity.

Modifying agroforestry to enhance biodiversity When a landowner or farmer considers an agroforestry planting, they usually go through a series of steps. First they examine their land and consider where a planting might go. Next there might be consideration of the size and shape of the planting, then what types of trees might be put in. The process takes into account any special needs of the planting and ongoing management considerations. At each step there are various factors to keep in mind: profitability, opportunity cost, accessibility and possible environmental outcomes. Enhancing biodiversity is one outcome that can be planned for and the following discussion outlines principles to consider when making

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CASE STUDY 1

Agroforestry pays its way at Danengate Danengate is a grazing property near Hamilton in Victoria. In 1947, Danengate had over 600 remnant eucalypts. By 1987, only 37 remained. In most respects Danengate was just like any other grazing property in the area: it was lacking in native cover, old trees were declining, there was heavy dependence on chemicals for pest control, and stock was vulnerable to cold weather events. In 1987 Danengate was purchased by Don and Jann Jowett, whose vision for the property included a range of tree plantations integrated into other grazing activities. Paddocks were surrounded by fenced corridor/shelter belt plantings (two to three rows wide), with some small circular woodlots (0.4–1 ha) established within the paddocks (see Figures 6.3 and 6.4). The shelter belts and woodlots now provide timber, shelter and biodiversity, while pastures and stock provide annual cash flow. With small paddocks, shelter is always close at hand and birds feed on insects and seeds in the nearby pasture. Based on observation, Don and Jann now make shelter belts four rows wide to improve their biodiversity value.

Figure 6.3: Aerial view of one of Danengate’s paddocks. The upper circular planting is manna gum and the lower circle is a variety of sheoaks. The block planting is blackwood. Agroforestry at Danengate is providing a range of ecosystem services and will become an important part of the farm’s income stream (photo by Don Jowett).

On average, 1–2 ha of timber trees have been planted each year since 1989. These include blackwood, black wattle, drooping sheoak, river sheoak, swamp sheoak, Monterey cypress, Mexican cypress, manna gum, mountain grey gum, Sydney blue gum, river red gum, spotted gum and radiata pine. The aim has been to plant 30% of the property to trees, with 5–10% permanently fenced off. To some, the Jowetts are mavericks, but their new ways are paying dividends. In 1987–92, pastures required regular spraying for cockchafers and red-legged earthmite. Since 1992 no pasture has been sprayed or has needed spraying. In 1989–90, all plantations required spraying for spitfires, leaf blister and looper caterpillars. Since then, trees on Danengate have not been sprayed


for insects, nor have they required it. Don and Jann conclude that their revegetation activities have improved the biological control of pest insects of pastures and trees. On 1 December 1987, an unusual weather event (snow, rain and wind) killed 50 000 sheep in the area surrounding the property. Today, sheep off-shears can be placed into any paddock on Danengate with safety. In the early 1990s, lamb losses were significant in the exposed western areas of the farm, which precluded the use of those paddocks for lambing. Today, ewes are lambed in all paddocks with few, if any, losses from the cold. Blocks of agroforestry can be seen as ‘green insurance’. No one wants to make a profit from insurance; no one wants to have their house burnt or their car written off in a crash. Like insurance, trees can reduce risk of damage from severe storms and even reduce the rate of bushfire spread. Overall, Don and Jann have no doubt that productivity is greater now than it was before areas of the farm were retired from grazing to grow trees. Their figures indicate a 320% increase in sheep numbers despite 33% of the original area of pasture being devoted to trees. On Danengate, ecosystem services are an important and nurtured source of farm income.

Figure 6.4: Well-fed, well-sheltered sheep feed under spotted gums at Danengate. In the distance is a shelter belt of mixed natives (photo by Rod Bird). Source: Bird and Jowett, ‘Danengate’, case study 4 in Race and Freudenberger (2003).

decisions on location, configuration, composition, complexity and management. This framework of principles is based on Salt et al. (2004), which also provides detailed discussions of most of the research examples cited. Location

The placement of agroforestry plantings greatly affects their habitat values. The most important consideration is avoiding the removal or decline of remnant native vegetation, including native grasslands.

The focus of most agroforestry is on the establishment of trees in agricultural landscapes where most of the original native vegetation has been cleared. Where trees are established on land previously used for farming, usually on exotic pastures, there is real potential for the planting to improve the biodiversity value of an area. When considering the location, it is vital to assess how the tree planting will relate to the remnant vegetation on or around the property. There are four principles to consider when discussing location: adjacency, connectivity, landscape context and the protection of waterways.

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Adjacency Agroforestry blocks will usually support a greater diversity of species if planted near existing remnants than if planted many kilometres from a sizeable remnant. Being close makes it easier for native animals and plants to access any resources offered by the tree planting. Surveys of blue gum plantations in Western Australia, for example, have shown that a range of birds, including some judged to be at risk, used plantations but that this mainly occurred where a plantation was close to patches of native vegetation (Hobbs et al. 2002). Adjacent plantations also offer some degree of shelter to patches of remnant vegetation. Being too close, however, carries the risk that the tree plantation might place other pressures on the patch of remnant vegetation. These pressures include competition for available water by plantation trees, the invasion of the remnant patch by plantation tree seeds (Lindenmayer and McCarthy 2001), encroachment of pests (Hobbs et al. 2002) and hybridisation with remnant vegetation. The impact of management strategies such as pesticide and herbicide sprays drifting into the remnant patch are additional pressures. Although there are no studies that indicate an optimal distance between a tree plantation and a patch of remnant vegetation (in terms of maximising its biodiversity value while minimising adverse impacts), a good precautionary approach would be to have the plantation separated from remnant vegetation by a buffer planting of a variety of local natives. Connectivity Agroforestry can also be used to reduce the isolation of remnant forest habitats by providing corridors and stepping stones to assist in the movement or dispersal of plant and animal species (Figure 6.5). Dispersal is often needed when a patch of habitat is burnt out and species must recolonise the burnt patch from an unburnt refuge. Dispersal is also essential for wildlife species that spend all or parts of their lives in more than one patch of habitat. For example, a radio-tracking study of how greater gliders used remnant vegetation and pine plantations found that while the animals largely stayed within the patches of remnant vegetation (eucalypt forest), they occasionally used the pine forest to move from patch to patch (Lindenmayer

Figure 6.5: Shelter belts connecting patches of remnant vegetation add connectivity to a landscape (photo courtesy of Greening Australia ACT/SE NSW).

and Pope 2000). This finding suggests that the pine forest provided some value to the gliders in connecting the various patches of native vegetation. Connectivity is about a species’ ability to move through a landscape. Many studies have shown that some animals can move through trees but will not cross open paddocks; structures like logs may assist an animal moving through a plantation (Curry 1991). Landscape context The landscape context of a planting is also important. Attributes of the broader landscape, such as percentage cover of native vegetation, will have a bearing on the likelihood of the planting being used by native animals and plants. For example, there are generally many more bird species in landscapes with a 70% cover of native trees than in landscapes with only 10% tree cover. Hence there are likely to be many fewer birds in a plantation surrounded by only a 10% cover than in one surrounded by a high percentage cover of native vegetation. A study in landscape size areas in the Northern Plains region of Victoria (Bennett and Ford 1997) found that the occurrence of woodland birds varied significantly between landscapes. The number of bird species was best predicted by total tree cover and the number of streams. The analysis indicated that woodland birds are sensitive to the total amount of native vegetation found in a landscape, and implied a substantial loss of species in landscapes that have been almost entirely cleared of woodland habitat (Radford et al. 2005).


Guidelines on location Consider a map or plan of a property where agroforestry is being considered. Mark in areas of high biodiversity value. These include patches of remnant native vegetation, rocky outcrops, isolated native trees and watercourses. Locate plantations so that they complement these areas: s

site plantations to complement existing remnant vegetation (e.g. protect or connect patches of native vegetation, or site the plantation close to native vegetation);

s

in the case of extensive plantations, incorporate patches of remnant vegetation within plantings;

s

establish plantations around a riparian zone to protect this important area;

s

join forces with your neighbours to control pests (e.g. foxes) and jointly create biodiverse plantings across property boundaries.

Protection of waterways In terms of biodiversity value, the edges of creeks, streams and rivers (the riparian zone) should receive high priority for revegetation since they form an interconnected system of natural corridors throughout a catchment (Lindenmayer and Peakall 2000), they often provide rich habitats for wildlife, and revegetation along streams has many other benefits for water quality, aquatic environments and the reduction of soil erosion. Farm forestry can play a role in protecting this valuable area by being used as a buffer zone around the edges of, but not within, riparian vegetation and corridors. High priority should be given to protecting and enhancing riparian zones before effort is put into other kinds of corridors. Configuration

The size and shape of a patch of remnant vegetation or a nature reserve can have an important influence on what animals and plants will be found in that patch, and has relevance to the design of an agroforestry planting. Size Large blocks of remnant vegetation (and possibly agroforestry plantings) generally support more species than do small blocks or remnants. This is because more resources, such as insects for birds (Zanette et al. 2000), are available in 10 ha than in 1 ha. Large blocks are also likely to sample a greater variety of microhabitats including depressions,

ridges, drainages and slope aspects, than a small patch on just one north-facing slope. Studies of wildlife in patches of remnant native vegetation have consistently shown that the bigger the patch the greater the range and number of animals it can support (Bennett et al. 2000; Lindenmayer 2000). While this has relevance to farm forests where bigger is better, size is not the only important factor. A large plantation of one species won’t necessarily provide greater biodiversity value than a smaller mixed planting. This is especially the case if the plantation is managed as an agricultural crop with high chemical inputs. What is contained in a patch of vegetation, its mix of ages and structures, is just as critical as size. However, all other things being equal, if there is a diversity of plant species, ages and structures, then size becomes an important factor. Large patches tend to be more diverse than small patches, but the habitat values of small patches and even of individual paddock trees should not be dismissed (Gibbons and Boak 2002). Research has documented a remarkable diversity and abundance of bats feeding on paddock trees in the Riverina of New South Wales and Victoria (Bennett and Lumsden 2003). Squirrel gliders have been found to feed in paddock trees as long as they are within the gliding distance (50 m wide) is being planned, it is recommended that a flight path should run through it. This should be about 4 m wide and oriented along the length of the belt, not the width. This will allow larger birds such as rosellas an access route through the dense vegetation to their food trees. Where possible, local plants should be used and a variety provided in terms of height and structure, such as tall fast-growing eucalypts, bushy wattles, pine-leafed sheoaks and native callitris pines, and low-growing shrubs and tussock grasses. If the shorter plants are on the outside and the taller ones in the middle, this will enhance the effectiveness of the shelter belt for wind protection and ensure that the smaller plants are not excessively shaded by the larger ones. Prickly dense shrubs should be planted to give protection for

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nesting birds; shrubs such as grevilleas, hakeas and blackthorn are ideal. These also give good protection from wind and discourage cats. Introduced fruit-bearing plants such as hawthorn, cotoneaster and pyracantha should be avoided as they produce abundant fruit in winter, providing a feast for starlings and currawongs. Starlings are fierce competitors with native birds for nesting hollows, and currawongs are predators of small birds. A shelter belt should be sited where it will complement and connect existing stands of remnant native vegetation. It should be located near water (rivers or farm dams). Any information on local hydrogeology should be considered. For instance, it may be useful to know whether a wide shelter belt can be used to intercept lateral flow of water coming off a bare hill. Woodlots

A woodlot and a large-scale plantation are similar in that they are usually established with commercial outcomes in mind and often focus on growing a single species. The difference is mainly one of scale, with a woodlot ranging in size from 1–50 ha and usually contained on one farm, whereas a large-scale plantation is often measured in hundreds of hectares (or more) and frequently covers more than a single farm. When it comes to biodiversity, the same suite of principles apply: where possible, local native species should be used, there should be structural complexity within the plantation and the plantation should be located to complement existing native vegetation. The application of these principles to woodlots and large-scale plantations differs from other plantings in that the commercial nature of the operation limits some of the choices, although planning for biodiversity is still possible. How this might be done in a large-scale plantation is discussed below. Because woodlots are smaller in scale, the agroforester often has more options in respect of location and species composition. The woodlot should be sited in the context of the whole farm, starting with a map and aerial photo of the land. Areas of high biodiversity value should be identified: patches of remnant vegetation, clusters of remnant trees, watercourses, rock outcrops etc. Native vegetation on adjoining land or along adjacent road-

Figure 6.8: Dams can add a valuable element of complexity to a plantation (photo by Sylvia Leighton).

ways should also be identified, as should existing plantings. Using a plastic overlay, possible plantation locations and configurations that augment these important elements to biodiversity should be mapped out. Plantations may protect, connect or enlarge existing blocks of native vegetation. In terms of connectivity, a plantation does not have to physically abut a patch of remnant vegetation to improve its connectivity, but it might serve as a stepping stone to other patches. Remember, isolation is a species-specific threat. For instance, sugar gliders cannot cross a treeless gap of greater than 70 m, but many different birds occupy patches of woodland separated by from other sizeable patches by 1 km. Some lizards and plants may need continuous habitat and multiple generations of dispersing offspring before they move into a new agroforestry planting. Anything that increases the structural complexity and patchiness of the woodlot and its surrounds is likely to improve the habitat value of the area for a wider range of animals and plants. A strategy that adds considerable biodiversity value over time is the establishment of diverse habitat plantings close to the plantation, preferably between the plantation and patches of remnant vegetation. Habitat plantings add patchiness to the area, connectivity to the plantation and protection to remnant vegetation, and will be retained during and after harvesting of the plantation. Larger and more diverse habitat plantings should have greater biodiversity value in terms of adding connectivity,


Figure 6.9: Zones of different management to suit areas of varying biodiversity value.

creating additional habitat and protecting remnant vegetation. In terms of managing a woodlot, any action that makes it more like a native forest or woodland has the potential to enhance its biodiversity value. This includes leaving prunings and thinnings on the ground, adding refuges such as hollow logs and establishing an understorey of native vegetation. This also applies to harvesting. Consideration should be given as to whether it is possible to selectively harvest the woodlot, taking out single trees for specific timber needs such as furniture-making. It might be possible to progressively add to the ‘naturalness’ of the plantation by varying the age and spacing of the stand while allowing for the retention of ground cover and the build-up of leaf litter, branches and logs. Large-scale plantations

As noted above, most large-scale plantations are established with commercial outcomes in mind, and this limits options in respect to biodiversity. It is better from a biodiversity standpoint to choose a mix of local native species planted with irregular spacings and long rotations, but this is rarely compatible with a commercial outcome. If biodiversity is a desirable outcome, a component of the plantation could be set aside and dedicated as an area for a habitat planting and the remainder of the plantation left for the commercial crop. Habitat plantings might be in the form of a central block, corner plantings or perimeter rows, and should be left intact when the plantation is har-

vested. Another option would be to plant belts of local native species throughout the plantation, such as three rows in every 40. Again, these areas should be left intact during and after harvesting. In a large-scale plantation, economies of scale dictate against selective harvesting practices but it might be possible to establish a harvesting regime that allows the creation of a patchwork of coupes of different ages. Areas around patches of remnant vegetation might be zoned for less-intensive harvesting, in which some mature trees might be left standing (see Figure 6.2). Harvesting could be planned so that some mature plantation is always present around parts of a remnant patch, as this improves connectivity between the remnant patch and the surrounding landscape. Harvesting practices that remove or destroy structures such as log piles reduce the biodiversity value of plantations, and harvesting practices that spread weeds from one area to another can have significant negative overall biodiversity conservation outcomes. Enhancing biodiversity values is about both thoughtful design at the beginning and responsive management over decades. Harvesting is a major disturbance that can be reduced by sensitive design implemented 30 years before harvesting. Habitat plantings

At the other end of the plantation spectrum are habitat plantings designed specifically to encourage a diversity of species and ecosystem processes. All the principles outlined so far apply, with the aim of implementing them to the maximum in

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Figure 6.10: Habitat plantings can add to the biodiversity value of a nearby commercial plantation. However, even with volunteer labour they can be expensive to establish (photo by David Salt).

order to enhance the existing elements of biodiversity in the landscape, restore a wide diversity of local plants established on appropriate sites, create large patches of quality nature plantings that are close to patches of remnant vegetation, connect various elements across the farm such as water points, old trees and watercourses, fence off native vegetation to protect it from stock, manage for weeds and feral pests, and coordinate efforts with others in the catchment to maximise the regional impact. It is easy to make a list like this, but it is much harder to implement. There may be no great expectation of a commercial return on the plantings, the establishment of new vegetation still costs time and money, and even with the most economical form of restoration planting the costs quickly accumulate. For example, we need to ensure that the area being planted is protected from grazing stock with fencing, which can cost several thousand dollars per kilometre. The most basic site preparation requires the reduction of biomass (usually grass) with the application of herbicide laid down in strips along planting lines. Direct seeding behind a tractor costs around $150/km or $375/ha, and although it can lay down significant quantities of seed, survival can be variable. Tubestock usually provides greater certainty but is more expensive and takes more labour in the planting. Tubestock seedlings usually cost around $2.50 each with an additional $0.50 for tree guards to protect the seedlings from wind and rabbits. Fol-

low-up inspections are needed to ensure the plantings are not being dominated by just a few species, or serving as a refuge for pests or weeds. These rough estimates demonstrate that even with volunteer labour and the best of intentions, the most basic restoration plantings still cost a lot of money. Indeed, this is why many state and Commonwealth government authorities are investing money in the search for native species that can be grown on a large scale with a commercial return. They are following this course because restoration and nature plantings simply cannot be undertaken on the scale necessary to address many of regional Australia’s environmental problems. Much research has been done on how to plan and manage effective revegetation, restoration and nature plantings (Bennett et al. 2000). There are a number of groups and organisations with expertise to help plan and possibly assist with restoration plantings. These include catchment management authorities, Greening Australia, Landcare and Land for Wildlife.

Agroforestry’s role in protecting biodiversity No matter what action is taken, land managers are always creating ‘winners’ and ‘losers’. Some native species thrive in monoculture blocks of commercial plantings of blue gums – they are called insect pests. Other species cannot find a long-term home in short-rotation plantings because they depend tree hollows that take hundreds of years to form into the right size and shape. The challenge of conserving biodiversity in production landscapes is to maintain or create patches of habitat for the species and processes that do not thrive in monocultures of annual crops, pastures or trees. Conservation of biodiversity is a whole-oflandscape challenge, not just a stand-level objective. Can agroforestry make a significant contribution to meeting this challenge? The answer is a qualified yes. Agroforestry can enhance elements of biodiversity if the patchiness or heterogeneity of landscapes is protected and enhanced. Patchiness applies to both space and time. Agroforestry can provide complementary habitat if diverse habitats are protected or rehabilitated, including patches of remnant forests, woodlands, grasslands,


CASE STUDY 2

Bringing the birds back to Lyndfield Park Lyndfield Park is a farm near Canberra. Over the last two decades, the property has undergone a dramatic transformation as its owners, John and Jan Weatherstone, have changed from growing sheep to growing trees. In the process they’ve turned around many of the chronic environmental problems threatening the farm, significantly increased its resilience to the stresses of drought, improved its financial turnover, dramatically increased its capital value and created an attractive and pleasant place to work. On top of this, Lyndfield Park has more species of native birds than any other property in the area. John and Jan changed the management direction of Lyndfield Park following a major episode of drought. At the height of the drought of 1982/83, they saw that despite applying what was considered best practice, they were working the land too hard and it had lost its resilience. The drought had exacerbated a host of problems such as poor soil, tree dieback, declining biodiversity and a loss of productivity. They believed that if Lyndfield Park were to have any future they needed to take greater care of the land (Figure 6.11).

Figure 6.11: Part of the 15 ha habitat planting at Lyndfield Park. It began as a planting around 20 remnant trees (photo by John Weatherstone).

In the years following the drought, John and Jan reduced grazing pressure (eventually they stopped grazing sheep altogether and went to grazing cattle), minimised soil cultivation, cut back the application of farm chemicals and established more than 80 000 trees and shrubs. Indeed, selling native trees, shrubs and seeds has become the main enterprise of Lyndfield Park. Cattle are still grazed but at higher (equivalent) stocking rates than prior to the 1982/83 drought and with significantly less impact on the land. Farm forestry, with a range of native trees for timber products, is also being developed, and John and Jan are running a number of trials of natives to test their growth and marketability. These include plantings of sheoak, red box, yellow box, red stringybark, red ironbark and black oak. The Weatherstones’ efforts to create habitat for local wildlife reflect several of the principles discussed in this chapter. They knew that the larger the area they could revegetate with native plants, the greater the chance of providing adequate areas of habitat to support some species of wildlife. They also knew that an area containing existing trees and native shrubs was a good place to begin. Consequently, in 1988 they set aside 15 ha in the middle of the farm in which to

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undertake a long-term habitat restoration project. This area was on light shale country and did not support many sheep, so they were not losing much income by setting aside the land. Also, the site contained 20 natural trees, some native grasses and a few native shrubs. Not much, but better than nothing. Over several years, John and Jan enhanced this area by planting a wide mix of native trees and shrubs. Diversity was the key: variety in plant species and variety in plant shapes and sizes – variety in what is available through the year. One of the ways they encouraged birds and other animals to use the farm was to have a variety of plants flowering at all times of the year. They now have at least one or two species of wattle flowering every month, as well as a range of other flowering trees and shrubs. Another important factor was establishing prickly shrubs in many areas, as these plants provide secure nesting sites for birds and a safe haven from predators. The results have been spectacular. Since 1996 just on 120 species of native birds have been identified on Lyndfield Park. Many of these species have not been recorded in the area before, and many others had disappeared during the previous few decades. For all this success, John and Jan acknowledge that there is one component of habitat that they cannot easily recreate. ‘Probably our greatest habitat deficiency is the lack of trees with hollows’, says John. ‘Regretfully, we removed most of the old trees long before we recognised their value. Now there’s little we can do in the short term to rectify that.’ Source: Weatherstone (2003).

riparian strips and wetlands. The habitat values of agroforestry can be enhanced if there are different kinds of plantings established in different parts of the landscapes at different times, with a wide range of harvesting rotations. Commercial values of agroforestry do not have to be sacrificed to enhance landscape patchiness. No landscape of any size, from a few hectares to hundreds of square kilometres, is uniformly productive or uniformly profitable in either space or time. Dry stony ridges will never be as productive as more fertile lower slopes. There is no guarantee that any one agroforestry species will be profitable and tolerant of climate change 20–50 years from now, when ready to harvest. Well-planned agroforestry that recognises the inherent variation in landscape productivity and aims to reduce long-term risk due to climatic variation should enhance the biodiversity of landscapes that may otherwise be dominated by a few land uses such as crops and pastures.

References and further reading Abel N, Baxter J, Campbell A, Cleugh H, Fargher J, Lambeck R, Prinsley R, Prosser M, Reid R,

Revell G, Schmidt C, Stirzaker R and Thorburn P (1997) Design Principles for Farm Forestry. Rural Industries Research and Development Corporation, Canberra. Barrett G (2000) Birds on farms: ecological management for agricultural sustainability. Wingspan 10(4), Supplement. Birds Australia, Melbourne. Bennett A and Ford LA (1997) Land use, habitat change and the conservation of birds in fragmented rural environments: a landscape perspective from the Northern Plains, Victoria, Australia. Pacific Conservation Biology 3, 244–261. Bennett A and Lumsden L (2003) Bats and Paddock Trees: Insights from Current Research. Department of Sustainability and Environment, Vic. Bennett A, Kimber S and Ryan P (2000) ‘Revegetation and wildlife’. Environment Australia, Bushcare Research Report 2/00. Binning C, Baker B, Meharg S, Cork S and Kearns K (2002) ‘Making farm forestry pay: markets for ecosystem services’. Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. 02/005.


Bird R (2000) Farm Forestry in Southern Australia. Pastoral and Veterinary Institute, Department of Sustainability & Environment, Vic. Bonham K, Mesibov R and Bashford R (2002) Diversity and abundance of some grounddwelling invertebrates in plantation vs native forests in Tasmania. Forest Ecology and Management 158, 237–247. Borsboom A, Wang J, Lees N, Mathieson M and Hogan L (2002) ‘Measurement and integration of fauna biodiversity values in Queensland agroforestry systems’. Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. 02/044. Catling P and Burt R (1995) Studies of the grounddwelling mammals of Eucalypt forests in south-eastern NSW: the effect of habitat variables on distribution and abundance. Wildlife Research 22, 271–288. Curry G (1991) The influence of proximity to plantation edge on diversity and abundance of bird species in an exotic pine plantation in northeastern New South Wales. Wildlife Research 18, 299–314. Dames and Moore NRM/FOR TECH (1999) ‘Integrating farm forestry and biodiversity’. Rural Industries Research and Development Corporation, Canberra. RIRDC Report no. 99/166. Doherty MD and Wright G (2004) ‘Preliminary assessment of the impact of the 2003 wild fires on the flora and vegetation of Brindabella National Park, Bimberi Nature Reserve and Burrinjuck Nature Reserve’. A report prepared for the Reserve Conservation Unit, New South Wales National Parks and Wildlife Service, Queanbeyan. CSIRO Sustainable Ecosystems, Canberra. Freudenberger D and Stol J (2002) SAND Farmscapes Project Report: Integrating Production and Biodiversity. CSIRO Sustainable Ecosystems, Canberra. Gibbons P and Boak M (2002) The value of paddock trees for regional conservation in an agricultural landscape. Ecological Management and Restoration 3, 205–210. Gibbons P and Lindenmayer D (2002) Tree Hollows and Wildlife Conservation in Australia. CSIRO Publishing, Melbourne.

Goldney D and Wakefield S (1997) Save the Bush Toolkit. Charles Sturt University, Bathurst, NSW. Grabham C, Klomp N and Robinson W (2002) The Influence of Remnant Trees Retained within Native Hardwood Plantations on the Diversity and Abundance of Birds. Department of Natural Resources and Environment, Wangaratta, Vic. Guijt I and Race D (1998) Growing Successfully: Australian Experiences with Farm Forestry. Greening Australia, Canberra. Grant J (1997) The Nestbox Book. Gould League, Melbourne. Hobbs R, Floyd R, Cunningham S, Catling P and Ive J (2002) ‘Hardwood plantations: quantifying conservation and environmental service benefits’. Rural Industries Research and Development Corporation, Canberra. RIRDC Publication no. 02/083. Kinross C (2004) Avian use of farm habitats, including windbreaks, on the New South Wales Tablelands. Pacific Conservation Biology 10, 180–192. Klomp N and Grabham C (2002) ‘A comparison of the avifaunal diversity on native hardwood plantations and pastureland in north-east Victoria’. Johnstone Centre, Charles Sturt University. Report no. 164. Klomp N, Grabham C and Herring M (2001) ‘Fauna study of Ettamogah Forest’. Johnstone Centre, Charles Sturt University. Report no. 154. Lindenmayer D (2000) ‘Islands of bush in a sea of pines’. Land and Water Resources Research and Development Corporation. Research Report 6/00. Lindenmayer DB and McCarthy MA (2001) The spatial distribution of non-native plant invaders in a pine-eucalypt mosaic in south-eastern Australia. Biological Conservation 102, 77–87. Lindenmayer D and McCarthy M (2002) Congruence between natural and human forest disturbance: a case study from Australian montane ash forests. Journal of Forest Ecology and Management 155, 319–335. Lindenmayer D and Peakall R (2000) The Tumut experiment: integrating demographic and genetic studies to unravel fragmentation effects. In Genetics, Demography and Viability

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of Fragmented Populations. (Eds A Young and G Clarke) pp. 173–201. Cambridge University Press, Cambridge. Lindenmayer D and Pope M (2000) The design of exotic softwood plantations to enhance wildlife conservation: preliminary lessons from the Tumut fragmentation experiment, southeastern Australia. In Nature Conservation 5: Nature Conservation in Production Environments: Managing the Matrix. (Eds J Craig, N Mitchell and D Saunders) pp. 44–49. Surrey Beatty & Sons, Sydney. Lindenmayer D, Claridge A, Hazell D, Michael D, Crane M, MacGregor C and Cunningham R (2003) Wildlife on Farms: How to Conserve Native Animals. CSIRO Publishing, Melbourne. Majer J, Recher H and Keals N (1999) Canopy arthropod faunas in fragmented agricultural landscapes. In Conservation of Australia’s Temperate Woodlands. (Eds RJ Hobbs and C Yates) pp. 235–247. Surrey Beatty & Sons, Sydney. McIntyre S, McIvor J and Heard K (2002) Managing and Conserving Grassy Woodlands. CSIRO Publishing, Melbourne. New B and England M (2002) Farm Forestry: Designing for Increased Biodiversity. Department for Environment and Heritage, Adelaide. Noss R (1990) Indicators for monitoring biodiversity: a hierarchical approach. Conservation Biology 4, 355–364. O’Neill G (1999) Renaissance on Lanark.Wingspan 9(1), Supplement. Birds Australia, Melbourne.

Platt S (2002) How to Plan Wildlife Landscapes. Department of Natural Resources and Environment, Melbourne. Race D and Freudenberger D (2003) Farm Forestry for Green and Gold: Australian Experiences of Linking Biodiversity to Business Solutions. Australian National University, Canberra. Radford J, Bennett A and Cheers G (2005) Landscape-level thresholds of habitat cover for woodland-dependent birds. Biological Conservation 124, 317–337. Recher H, Shields J, Kavanagh R and Webb G (1987) Retaining remnant mature forest for nature conservation at Eden, NSW: a review of theory and practice. In Nature Conservation: The Role of Remnants of Native Vegetation. (Eds D Saunders, G Arnold, A Burbidge and A Hopkins) pp. 177–194. Surrey Beatty & Sons, Sydney. Salt D, Lindenmayer D and Hobbs R (2004) Trees and Biodiversity: A Guide for Australian Farm Forestry. RIRDC, Canberra. Smith G and Agnew G (2002) The value of ‘bat boxes’ for attracting hollow-dependent fauna to farm forestry plantations in southeast Queensland. Ecological Management & Restoration 3, 37–46. Taws N and Streatfield S (2001) Bringing Birds Back. Greening Australia, ACT and SE NSW. Weatherstone J (2003) Lyndfield Park: Looking Back, Moving Forward. Land and Water Australia, Canberra. Zanette L, Doyle P and Tremont S (2000) Food shortage in small fragments: evidence from an area-sensitive passerine. Ecology 81, 1654–1666.

7 Environmental risk in agroforestry Margaret Byrne, Lynley Stone and Melissa Millar

Introduction The development of agroforestry systems promises significant environmental and economic benefits, particularly in the management of dryland salinity. Large-scale planting of woody crops also poses some risks to existing natural biodiversity. These risks include the establishment of new plant species as environmental weeds, hybridisation with native species and gene flow from cultivated populations into natural populations. Agriculture and agroforestry don’t occur in isolation. They are dominant parts of many rural landscapes, having replaced native communities over large areas of southern Australia. Local and regional biodiversity are strongly influenced by human land use and its alteration in many landscapes. Chapter 6 discussed the biodiversity benefits agroforestry can provide. These benefits are most advantageous when agroforestry is integrated into the landscape. This integration also requires the recognition of any negative impacts agroforestry may have on biodiversity in the landscape. As pointed out in Chapter 5, biodiversity is complex and has both structural and functional aspects, essential for the integrity of complex self-sustaining natural ecosystems. Biodiversity integrates biotic variability at all levels of organisation including genes, organisms, populations, communities and ecosystems. One community is not the same as any other even though on a superficial level the same species composition may be present. Organisms have differentiated their functional traits and

niche occupation during speciation. Their coexistence is a reflection of functional speciation and niche complementarity (Beierkuhnlein and Jentsch 2005), particularly in southern Australia where current landscapes have been continuously occupied for millions of years due to lack of major glaciation events (Hopper et al. 1996). It is extremely difficult to put a value on native biodiversity, particularly in a world driven by economics. In agricultural regions in Australia, the value of biodiversity in maintaining functional landscapes is readily seen in the extent of hydrological imbalance leading to dryland salinity as a consequence of land clearing. However, biodiversity contributes to the human experience in many ways – consumptive use, productive use, opportunity use, ecosystem services, amenity values, scientific and educational experience, recreational opportunity and spiritual/philosophical experience (Wallace et al. 2003). Apart from the cultural, economic and ecosystem values that influence human experience, biodiversity has an intrinsic value (SEAC 2001). The first priority for biodiversity conservation is to protect existing biodiversity assets from further threat, the second is to enhance their condition and the third is to restore their former extent (Anderson et al. 2001). Biotic variability is fundamental for adaptation to the challenging environment of southern Australia, which in turn is critical to long-term survival. The ability to change allows functional stability and redundancy, and ensures persistence 107

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of ecosystems under changing environmental conditions and resilience in response to disturbance (Beierkuhnlein and Jentsch 2005). Two primary components of biotic variability are species diversity and genetic diversity. Plant species diversity plays a significant role in the control of ecosystem processes and overall functioning, and there is a close relationship between particular ecosystem processes and species diversity (Beierkuhnlein and Jentsch 2005). In some cases the effects will be related to complementarity of functional traits of species, in others just to the occurrence of key species. Species are not similar; the historical and evolutionary background of each species may have a strong influence on the performance of entire ecosystems (Beierkuhnlein and Jentsch 2005). There are various hypotheses on the role of species in ecosystem function: one end of the spectrum views each species as uniquely contributing to the integrity of communities (Ehrlich and Wilson 1981) and the other expounds that species diversity includes redundancy, in that ecological processes can be maintained by a subset of species (Lawton and Brown 1993). Regardless of which view is held, biological diversity is threatened by loss of species since there is no obvious way to identify keystone species a priori, redundancy is not easily recognised and there is no guarantee that species lost will have cooccurring analogues (Frankel et al. 1995). Genetic diversity is closely linked to adaptive potential, and the long-term survival of functional ecosystems depends on the genetic integrity of species and populations. Natural vegetation is a complex and finely tuned system. Disturbance of genetic and species diversity in communities impacts on a wide range of ecosystem functions (Frankel et al. 1995), affecting not only native vegetation but all elements of rural landscapes. Agroforestry provides major ecosystem benefits and, when integrated into landscapes, enhances the biodiversity values of the region. Are there any negative aspects from the agroforestry enterprise? Two factors that impact on the structural and compositional features of biodiversity include weed invasion and genetic contamination. Invasion by weeds changes the structural and compositional aspects of natural vegetation. Invasive species have been identified as the second-greatest threat to biodiversity, after habitat destruction (Walker and Steffan 1997).

Each year Australia faces a cost of over $4 billion from weeds and their impacts on agriculture, natural environments, public and indigenous land (Sinden et al. 2004). The fundamental basis of species composition is genetic diversity, and genetic contamination or hybridisation disrupts genetic integrity of species. This chapter discusses these factors of weed invasion and genetic contamination, their potential to negatively impact on remnant vegetation, and the opportunities to manage these impacts. There are also environmental risks from agroforestry in relation to sources of disease and habitat for pests. Although those aspects are not considered here, the principles of risk assessment and management discussed would apply to the evaluation of pest and disease impacts. Environmental risks can be managed; the critical aspect is to have sufficient information to assess and evaluate the risk, and to design management guidelines.

Weed invasion What is a weed?

Auld et al. (1987) summarised the definition of a weed as something that is both time- and locationspecific. It is a non-scientific term that centres on a human value judgement (Sindel 2000). That is, many weeds are also considered useful plants and are highly valuable in particular land use systems. For example, in forestry Pinus species are valued as a source of fast-growing softwood, but in natural ecosystems are invasive and are considered environmental weeds. Weed species are often pioneer species, meaning they are the first to colonise disturbed soil. In their native environment, other factors such as competition for resources, climate, presence of insects and disease would have controlled their spread and abundance (Sindel 2000). In a new environment, without these elements, these species can colonise new areas and invade native vegetation. Source of weeds

The majority of weeds in Australia are exotic, and many were purposefully introduced for horticulture, agriculture or garden/amenity use. There are more exotic taxa in Australia (approx. 27 000) (Virtue et al. 2004) than native species (15 638)

7 – Environmental risk in agroforestry

Table 7.1: Impact of naturalised flora on agricultural and natural ecosystems

Category Naturalised in Australia

No. of species affecting agricultural systems

No. of species affecting natural ecosystems

No. of species affecting both systems

~2700

~2700

~2700

Considered a major problem

426

798

276

Considered a minor problem

840

1388

533

Species harbour pests/diseases or are toxic or are having direct impact on threatened flora

295

49

7

25

34

7

Recommended for eradication and/or under eradication Source: Data from Groves et al. (2003)

(Hnatiuk 1990). Of course not all introduced plants become weeds. In fact, the majority do not impact negatively on biodiversity and provide many benefits to society through agriculture, horticulture, forestry, landscaping etc. However, a study of Australia’s naturalised flora published in 2003 revealed that of the 27 000 introductions approximately 2750 have naturalised (formed self-sustaining populations) and are having an impact on natural ecosystems or agricultural systems (Table 7.1) (Groves et al. 2003). Weediness is not restricted to exotic species, and there are an increasing number of examples of indigenous flora becoming environmental weeds outside their native range. For example, Acacia pycnantha from eastern Australia has become a weed in Western Australia, Tasmania and New South Wales, and overseas in South Africa. Acacia saligna is invasive in areas outside its native range in Western Australia and is known as an invasive weed in Chile, Spain, Portugal, Cyprus and South Africa (Maslin and McDonald 2004). Other native genera used in agroforestry that have been identified as having weed potential are Melaleuca and Casuarina (Marcar and Crawford 2004). Weed impacts in Australia

The increasingly fragmented landscape of Australia means that our ecosystems are at higher risk of invasion. In the wheatbelt region of Western Australian less than 10% of the native vegetation remains (Hobbs et al. 1993), with as little as 2–3% in some areas (Hobbs 2001). Australia-wide, environmental weeds are the presumed cause of the extinction of at least four taxa, with a further 57

species under threat from competition with weeds (Leigh and Briggs 1992). Many threatened flora exist in fragmented populations, and have a greater risk of invasion due to large edge:area ratios (Hobbs 2001). Ecosystems are more likely to be invaded by weed species if they have a low level of plant cover. In Western Australia, woodland communities are more susceptible to invasion than shrub communities, as they have little understorey and have higher soil nutrient levels (Hobbs et al. 1993). Soil disturbance, human or machinery disturbance and even frequent fires provide opportunities for invasion. Hobbs and Atkins (1988) found that seeds from annual weeds were able to penetrate large patches of remnant vegetation, but did not necessarily germinate and survive without some form of disturbance. If disturbance was accompanied by the addition of nutrients, weed survival increased. Remnant vegetation often occurs in close proximity to areas of agricultural production, which contributes to two of these factors – they provide a large source of immigrant plants and the addition of nutrients to the soil is commonplace. The impact of weed species on ecosystems varies, but major impacts include (Williams and West 2000):

s s s s s s

competition for light, nutrients, moisture and pollinators; replacement of indigenous plant communities; prevention of natural regeneration; change in movement of water in soil and watercourses; increased soil erosion; provision of food and shelter for pest animals;

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s s s s

reduced water quality; introduction of foreign genes into local plant populations; altered fire behaviour and fuel loads; altered disturbance regimes.

Invasion by weeds does not affect only plant species in an ecosystem. Changes in the structure and composition of an ecological community may also have an impact on the vertebrate and invertebrate fauna of that area. For example, Mimosa pigra forms impenetrable thickets in tropical Australia, replacing riverine and floodplain vegetation, including sedgeland, paperbark forest, monsoon forest and riparian woodland. This invasion has reduced the abundance of birds and reptiles in the area and is predicted to cause a decline in mammal populations as food sources disappear (Braithwaite et al. 1989). Weed threats in agroforestry

A review of agroforestry species planted worldwide suggested that approximately 1% were considered weedy in more than half the records surveyed and a further 7% were weedy under certain conditions (Richardson 1999). Characteristics desirable in an agroforestry species, such as rapid growth and establishment, and adaptability to soil and climate, also enable a species to become invasive. Pinus spp. are particularly weedy in both the northern and southern hemispheres, and there has been much research to assess the impacts on native ecosystems where wildlings have invaded (Richardson 1998). In his review, Richardson (1999) noted that 19 Pinus species are invaders of native ecosystems in the southern hemisphere. A large study conducted in New South Wales (the Tumut fragmentation experiment), revealed Pinus radiata wildling occurrence increased in eucalypt forest where there was limited ground cover and where remnant forests had been surrounded by pine stands for a prolonged period (Lindenmayer 2000b; Lindenmayer and McCarthy 2001). In South Australia, Pinus brutia, P. halepensis and P. radiata have been identified as species with high to very high weed risk (Virtue and Melland 2003). High seedling densities resulting in dense thickets under parent trees and a build-up of pine needles in the ground litter affect native vegetation by inhibiting native seed-

ling establishment and reducing the growth of shade-tolerant native species. Acacia species are also notorious worldwide for their invasive characteristics and appear, with Pinus, on a large number of weed lists (Richardson 1999). Acacia saligna is a species with high potential for agroforestry in southern Australia but has been identified as having high weed risk (see case study). However, there is variation in A. saligna for suckering, a characteristic favourable for weediness. Use of non-suckering variants or selection of non-suckering lines may overcome the weed risk in use of A. saligna. This highlights the importance of knowledge of species biology in utilisation. There are other examples of agroforestry species becoming invasive, such as the olive (Olea europaea subsp. europeae) and Leucaena leucocephala. Olives can spread prolifically through the wide dispersal of seeds by birds that eat the fleshy fruits. Dense stands form a permanent crown under which native plant species cannot survive or regenerate, resulting in displacement of native species and dramatic changes in biodiversity (Spennemann and Allen 2000). The invasibility of these species has been recognised by industry and a proactive approach to management has been taken. Olive trees, other than those actively cultivated, are proclaimed as weeds in South Australia and there is a code of practice that growers must follow when cultivating olives (Anon 1999). A code of practice has also been adopted in Tasmania to prevent the naturalisation of the species in habitat that would provide ideal growing conditions. Leucaena leucocephala is extensively used in Queensland as a highly valued palatable forage tree for beef production. It is a prolific seeder and is highly invasive, and has potential to become a serious weed of riparian zones in Queensland. Growers in Queensland have adopted a code of practice and the Queensland government has developed a policy for reducing the weed threat of Leucaena (Anon 2004b).

Genetic contamination Genetic contamination describes the movement of foreign genes from domesticated or other non-local populations into native populations via pollen (Arnold 1992). This may occur when non-local species have been introduced or the habitat


CASE STUDY 1

Acacia saligna The species Acacia saligna is highly variable in morphology and is being reclassified into a number of subspecies (Maslin and McDonald 2004). s

Subspecies lindleyi is widespread and occurs predominantly in the wheatbelt region of Western Australia. It grows as a shrub or small tree with usually stout, straight to substraight stems, smooth bark and green phyllodes. This subspecies suckers, but the predominant mode of spread appears to be by seed.

s

Subspecies saligna is found along the Swan coastal plain near Perth and on the south coast near Esperance. It grows as a tall erect shrub or tree with robust straight to substraight stems, grey-green to subglaucous phyllodes and smooth bark, although the bark becomes longitudinally fissured in older plants. Suckering is infrequent.

s

Subspecies stolonifera occurs in the southern forest region and grows as a tall shrub or tree with substraight to crooked stems and friable bark. It appears to sucker aggressively.

s

Subspecies pruinescens is geographically restricted and largely occurs within the range of subspecies stolonifera. It grows as a tree with straight stems and friable bark, tends to have white pruinose branchlets and relatively short glaucous phyllodes. It suckers strongly.

The status of a possible fifth subspecies of limited distribution is being assessed. Analysis of the genetic diversity within A. saligna showed significant differentiation into three genetic groups, which corresponded with subspecies lindleyi, stolonifera and saligna/pruinescens (George et al. 2006; Millar et al. 2008).

Uses A. saligna was ranked as the species with the best potential for utilisation as a crop species in southern Australia by the AcaciaSearch assessment of 35 species of Australian acacia (Maslin and McDonald 2004). A. saligna is a versatile plant, and efforts are underway to improve its value as a fodder species by selecting for low tannin content and increased digestibility for stock. As a timber it is used for fuelwood and charcoal, and it shows promise for particle board (Bartle et al. 2004). It has been used to provide screening, windbreaks and shade for stock and wildlife, and has gained attention for its use in mitigating salinity (Maslin and McDonald 2004). Fast growth and survival has been recorded in saline and waterlogged clay soils (Bennett and George 1993).

The risk to biodiversity The weed potential of A. saligna outside its native range is well-documented. Naturalised populations can be found in Victoria, South Australia, New South Wales and Queensland. It has also become invasive in South Africa, Chile, Spain, Portugal and Cyprus (Maslin and McDonald 2004). In South Australia, A. saligna is classed as a medium- to high-risk species (Virtue and Melland 2003) due to its rapid growth, difficulty in distinguishing it from indigenous Acacia spp. (particularly as seedlings), and long-lived soil seedbank. Trees readily sucker and resprout from cut stumps, and repeated herbicide application is required for eradication.

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Overlap in flowering times between A. saligna subspecies means that there is high risk of genetic contamination if one subspecies is planted within the natural range of another subspecies in Western Australia. The high level of genetic differentiation in A. saligna also means that the impacts of genetic contamination may be significant for natural populations. Gene flow studies have confirmed hybridisation in 14% of subspecies saligna progeny via genetic contamination from trees of subspecies lindleyi (Millar and Byrne 2007). Genetic contamination in subspecies lindleyi from subspecies saligna was detected at over twice that rate (32%) at the same site (Millar et al. 2007). While the majority of genetic contamination occurred at short distances of 1.5 km may be required in certain circumstances (Millar et al. 2007; Millar and Byrne 2007).

Figure 7.1: A. saligna.

Figure 7.2: Phyllodes of A. saligna.


modified, bringing previously isolated species or populations into geographic contact. Gene flow is an important factor in natural population and ecosystem processes and is usually considered beneficial to plant conservation (Ellstrand and Elam 1993). It shapes gene pools and the population genetic structure of species, acting as a force to maintain genetic continuity between populations and preventing the loss of genetic diversity through inbreeding. However, gene flow can be negative when it occurs between divergent populations, as it may result in hybridisation. Hybridisation is typically defined as interbreeding of individuals from genetically distinct populations, regardless of their taxonomic status. While hybridisation is readily recognised as a result of gene flow between species or subspecies, gene flow between differentiated gene pools within species can be thought of as intraspecific or interprovenance hybridisation. Concerns about the impacts of hybridisation in conservation management are increasing (Ellstrand et al. 1999; Potts et al. 2003; Rhymer and Simberloff 1996; Strauss 2001). Hybridisation can result in one of two outcomes for hybrid progeny – heterosis, or outbreeding depression (Ellstrand and Elam 1993). Heterosis, also known as hybrid vigour, occurs when hybrid fitness is enhanced relative to the parental populations. The vigour of an initial hybrid generation may result in the establishment of hybrid populations between domesticated and natural populations. If the hybrids maintain increased fitness relative to the parental populations through successive generations, the typically smaller native population is at risk from a loss of genetic integrity. This may eventually result in total genetic assimilation via introgression. Introgression is the transfer of genes from one population to another by the repeated backcrossing of hybrids to one or both parental populations. It may result in a small component of the donor genome being transferred to the native gene pool or may extend to virtually complete introgressive displacement of the native population. Such displacement, or genetic assimilation, may result in extinction of native populations in the absence of selection against hybrids. Native populations are likely to suffer from direct competition and reduction in size and be subject to the impact of small

population processes. If the growth rate of native populations declines below that required for replacement, they may become extinct due to hybrid swamping. An alternative outcome from hybridisation is outbreeding depression, which occurs when hybrid fitness is reduced relative to the parental population. The extreme case of this is when all hybrids produced are sterile. In this case the reproductive output of a native population may be lowered such that it leads to extinction (Rhymer and Simberloff 1996). This effect will be exacerbated when pollen swamping is extreme, the reproductive output of the population is low or the population is already subject to the impacts of small population processes such as inbreeding. Impacts of genetic contamination

The impacts that genetic contamination or hybridisation from agroforestry species may have on native populations are difficult to predict but are related to:

s s s s

the scale of genetic divergence between the domesticated and native populations; the nature of the evolutionary process generating the divergence; the size of the pollen source and sink populations; the functionality of the ecosystem.

Scale of genetic divergence Native agroforestry species have undergone little selective breeding for domestication to date (Adams and Burczyk 2000), particularly if a broad base of genetic diversity has been captured, as is generally the case in tree breeding programs (Moran et al. 2000). Breeding itself is not a significant factor leading to differentiation of planted populations. However, native species under utilisation are generally widespread species that occur across broad environmental gradients, as these are characteristics that increase their utility. It is now recognised that many of these species are species complexes where the morphological and genetic variation has not been resolved taxonomically, particularly in south-west Australia, where a long evolutionary history in a stable landscape has led to a high degree of morphological variation that contributes to a

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certain degree of taxonomic ambiguity (Coates 2000). Combined genetic and taxonomic studies of key species identified as having potential for crop development are being undertaken to provide more detailed understanding of taxonomic entities and their genetic relationships. Most studies of native species have revealed some genetic divergence but there are exceptions, for example there was little genetic differentiation in the oil mallee Eucalyptus kochii (Byrne 1999). A high level of divergence was observed between the variants of A. microbotrya (Byrne unpub.) and A. saligna (George et al. 2006) and a phylogenetic study of populations of Melaleuca uncinata confirmed the presence of genetic divergence between groups that have since been recognised as new species (Broadhurst et al. 2004). Differentiation has also been identified between eastern and western regions of widespread species in northern Australia, e.g. E. camaldulensis (Butcher et al. 2002), A. auriculiformis (Wickneswari and Norwati 1993) and A. tumida (McDonald et al. 2003). The identification of significant levels of divergence and presence of unrecognised taxa within such species indicates that their use as agroforestry crops involves some risk of genetic contamination via hybridisation with native populations. A clear definition of the taxa in key agroforestry species complexes is required for more certain identification and characterisation of native populations. This will enable agroforestry management practices to ensure that divergent populations are not planted in close proximity to each other. Evolutionary processes that generate divergence The impacts of crossing between differentiated populations depend on the nature of the genetic processes underlying the divergence between source and sink populations. If divergence is due to selection and adaptation, outbreeding depression can occur, depending on the nature of the population adaptation (Templeton 1986). If population fitness has occurred through selection for locally adapted genotypes then hybridisation will result in the dilution of these genotypes, as hybrids show heterozygosity and underdominance at loci formerly fixed for adapted alleles. If the population fitness occurs through the development of coadapted gene complexes (distinct combinations of epistatically interacting loci), hybridisation may

expose these complexes to disruption through recombination. The effects of disruption to coadapted gene complexes are not expressed until the F2 generation and are often seen as advanced generation breakdown (Fenster and Dudash 1994; Hufford and Mazer 2003; Templeton 1986). Size of pollen source and sink populations The risk of genetic contamination from planted populations depends greatly on the relative size and distribution of native and domesticated populations. In Australia’s disturbed agricultural landscapes, broad-scale revegetation for agroforestry may generate large amounts of contaminant pollen (source) in comparison to small natural pollen pools (sink). As native population sizes decrease, the relative fraction of immigrant pollen increases (Ellstrand and Elam 1993) and the impacts of genetic contamination are greater. For example, small populations of the rare E. aggregata that are outnumbered by the common E. viminalis and E. rubida show high levels of hybrid seed production compared to large populations (Field et al. 2004). The effects of large-scale changes in gene flow are likely to have negative consequences for the long-term persistence of small native populations, especially as remnants may already be suffering from the effects of small population size. Ecosystem functionality The fragmented agricultural landscapes of southern Australia are examples of disturbed landscapes where ecosystem processes have been highly disrupted. Such disturbed landscapes are typically composed of small remnant populations, where gene flow is especially important in shaping genetic structure. In small, genetically or geographically isolated populations, effective population sizes may become limited so that the effects of inbreeding depression (loss of fitness due to increasing homozygosity) produce a significant decrease in reproductive performance (Ellstrand and Elam 1993). This is especially relevant to predominantly outbreeding species, such as most trees, where significant increases in inbreeding can lead to depleted genetic variation within populations and increased differentiation among populations. Decreased genetic variation has negative effects on population responses to selection pressures


caused by changing environments, and ultimately on population persistence. The impact of gene flow between planted and natural populations would be less in functional ecosystems with large native populations. Ecological instability brought about by disturbance also tends to create heterogeneous habitats which provide greater opportunity for hybrid establishment (Stebbins 1959). The risk of hybridisation through pollen dispersal increases with fragmentation and reduction in population size. Investigation of the mating system in E. benthamii, a rare species in the Camden area of New South Wales, showed that small populations had 25–35% hybridity in their seed crops arising from hybridisation with nearby E. viminalis trees, compared to larger populations where there was no evidence of hybridisation (Butcher et al. 2005). Hybrid zones may lead to increased herbivore and pathogen pressure in native vegetation. Hybrids are often more vulnerable to pests and may support considerable and diverse pest assemblages in close proximity to the parental species (Dungey and Potts 2001; Whitham et al. 1999). These may serve as staging areas for pest establishment on parental species.

Assessing the risk Risk assessment is the combination of determining how likely an event is to happen, and the consequences of that event. In the context of this discussion, the question to ask is: ‘What risk does agroforestry pose to biodiversity?’ The answer will include an evaluation of:

s s

the risk of agroforestry species invading and impacting on native ecosystems; the risk of gene flow from cultivated plants to indigenous plants of the same or related species.

Assessing weed risk

Predicting invasiveness of a species The ability of a species to become invasive depends on a number of variables. One philosophy of predicting invasive characteristics is that all species have the traits necessary to invade, as they must have the ability to persist (even if at low numbers) or they would become extinct. However, weed spe-

cies vary widely in their invasive capacity and impact on ecosystem processes (Heywood 1989). All introduced species affect the communities in which they occur, but invasive and aggressive species have significant impacts on communities. If habitat and environmental conditions are ideal – an ‘invasion window’ presents itself (Johnson 1986) – then any plant species has the potential to become invasive. For some species, called ‘sleeper weeds’, this window may not occur for many years. In Florida, the native Australian species Melaleuca quinquenervia was introduced in the early 1900s as an ornamental and erosion control tree, but it wasn’t until decades later that it was recognised as a serious environmental weed infesting over 200 000 ha of wetlands (Turner et al. 1998). In Germany, the average time from the planting of an exotic shrub or tree to recognition as an invasive species was 131 years for woody shrubs and 170 years for trees (Kowarik 1995). An Australian example of a woody sleeper weed is Tamarix aphylla, also known as Athel pine or tamarisk. Athel pine was introduced to Australia in 1930 and was extensively planted for shelter, shade and erosion control in South Australia. It wasn’t until the 1970s and 1980s, when extensive flooding occurred, that its distribution greatly increased along waterways throughout inland Australia and by the late 1980s it was formally recognised as a weed (Anon 2003; Griffin et al. 1989). In 1999, T. aphylla was declared as an Australian Weed of National Significance (WONS), one of 20 species identified as posing the greatest threat to socioeconomic and environmental values (Thorp and Lynch 2000). The biological attributes of invasive species vary so widely that many believe that biological attributes alone are a poor indicator of invasiveness. There are particular characteristics that assist a species to become invasive (Table 7.2), and many of these attributes have been incorporated into weed risk assessment protocols to try to predict whether a species will become weedy. Table 7.3 shows some of the biological attributes of woody species identified as WONS, and woody species that are on the environmental weed alert list. A comparison of the characters in Tables 7.2 and 7.3 shows many of the key characters associated with invasiveness are present in Australia’s worst woody

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Table 7.2: Ecological attributes that increase weed potential in native ecosystems High input of viable propagules Short (5 yr) periods of dormancy High rate of aerial or subterranean biomass production, particularly under conditions of low light, water or nutrient availability Dense and spreading foliar canopy Efficient long-distance (>1 km) dispersal capabilities Presence of interspecific allelopathic properties or absence of intraspecific allelopathic properties Successful colonisation of disturbed or bare ground Reproductive strategies that facilitate survival in fire-prone environments Broad distribution over a range of distinct climatic types Low susceptibility to attack by phytophagous organisms Source: Adair (1995)

weeds. For example, for the 18 species listed most use water to disperse seed, enabling long-distance dispersal, the maximum time to flowering in the species is only three years, most of the species produce many thousands of seed annually and only two species do not have dormant seed. Environmental weeds are not specific to a few plant families and examples can be found in most families of plants. In North America the Rosaceae (16%) have the most weed species (Reichard 2001), in New Zealand, the families with the most weed species are Poaceae (13%) and Fabaceae (8%) (Williams & West 2000), and in temperate Southern Australia, Asteraceae is the family with most weed species (Groves & Burdon 1986). The single best predictor of invasive ability is knowledge of species behaviour in other areas where it has been introduced (Daehler et al. 2004; Reichard 2001; Rejmanek 2000). In North America for example, 54% of invasive species invade elsewhere, and in Hawai’i, 76% of invasive species are invaders elsewhere (Reichard 2001). Weed risk assessment protocols The ability to assess the risk of a plant becoming a weed is important for weed management. Weed risk

assessment is ‘the use of standard, technical criteria to determine the relative weed threats posed by different plant species’ (Virtue et al. in press). Over the years, several efforts have been made to produce an effective weed risk assessment protocol to enable a scientific assessment of a plant’s capacity to become a weed. Two types of assessment protocols have emerged – predictive and prioritising. A predictive weed risk assessment aims to predict the risk of a plant becoming a weed a priori, i.e. before it happens. A prioritising assessment aims to identify the impacts of weed species and potential for containment, to prioritise species for management. Biosecurity Australia is the national government agency charged with developing policy to control entry of species into Australia, which is then implemented by the Australian Quarantine Inspection Service (AQIS). AQIS uses a predictive weed risk assessment protocol to assess whether plants species being imported into Australia could become environmental or agricultural weeds. Any plant species deemed to be of high risk is not permitted entry to Australia. This system has been adapted for use in New Zealand (Pheloung et al. 1999) and Hawaii (Daehler et al. 2004), and is internationally recognised as the most accurate and effective system available. The weed risk assessment protocol consists of 49 questions divided into eight sections: 1 2 3 4 5 6 7 8

domestication/cultivation; climate and distribution; weed elsewhere; undesirable traits; plant type; reproduction; dispersal mechanisms; persistence attributes.

The answers to the questions generate a score (usually +1 for a weedy attribute and -1 for a nonweedy attribute) which is converted into a recommendation of ‘accept’ (total score ≤0), ‘evaluate’ (total score 1–6) or ‘reject’ (total score >6). Species classed as ‘evaluate’ require a wider literature search or collection of further data from glasshouse or field trials to determine the potential weed risk. Other weed risk assessment systems in use by state agencies in Australia aim to prioritise weeds for cost-effective control or eradication. These protocols use questions similar to the Biosecurity


Australia model, but the outcome enables species to be prioritised using local data. This type of assessment usually assesses the invasiveness of a species, its impacts and potential distribution. There is no uniform process used by all Australian states, but a national post-border weed risk management protocol published by Standards Australia aims to harmonise post-border models (Standards Australia 2006). The South Australian weed risk assessment protocol (based on an earlier draft of the national protocol) was used by Virtue and Melland (2003) to evaluate the environmental weed risk of revegetation and forestry species in South Australia. Similarly, in Victoria, a Pest Plant Prioritisation Process, also based on an earlier draft of the national protocol, has been used to review the Victorian noxious weeds list (Weiss et al. 2004). Assessing genetic risk

No predictive framework exists to assess the likelihood that hybridisation will lead to extinction, the rate with which it may occur or the quantitative assessment of the relative importance of risk factors. Considerable genetic and ecological information is required to predict the likely outcome of hybridisation between species. Effective risk assessment and the development of management guidelines for the use of native species in agroforestry requires key research in the following areas:

s s s s s

crossability of key species; spatial patterns of genetic diversity; distances and levels of gene flow; long-term monitoring of introgression; impacts on local biodiversity.

Crossability of key species While the natural distributions of key agroforestry species and their relatives are generally well-known, further information is required regarding the likelihood of hybridisation. Studies of natural and manipulated hybrids and knowledge of barriers to hybridisation, including pre-zygotic and postzygotic barriers, will provide further information on the potential of hybridisation with related species in intended planting areas (Potts et al. 2001). This information must be associated with ongoing changes in taxonomy. Knowledge of flowering phenology of closely related species in different environments is crucial

as this is often the first barrier to hybridisation. Flowering phenology is readily assessed, although flowering time is influenced by environment and can vary markedly across different environments. Potential for crop–wild hybridisation, including forestry crops, has recently been assessed at a national level in New Zealand (Armstrong et al. 2005) and potential for hybridisation between exotic E. nitens and native eucalypts in Tasmania has also been assessed (Barbour et al. 2005). Spatial patterns of genetic diversity Many of the native species identified as having potential for agroforestry are species complexes where morphological and genetic variation has not been fully resolved. Combined genetic and taxonomic studies of these species are required for a more detailed understanding of taxonomic entities and their genetic relationships. This will allow for more efficient processes of identification and characterisation of germplasm resources for use in domestication programs. It will also help to ensure that the environmental and economic impacts of agroforestry can be achieved without creating additional risks to existing natural biodiversity. Distances and levels of gene flow Pollen dispersal patterns should be investigated for key agroforestry species likely to be in contact with vulnerable populations capable of hybridisation. This will allow for the description of recommended safe isolation distances and buffer zones. Pollen-mediated gene flow in plants is nearly always described by a leptokurtic distribution of pollen from its source, with the majority of pollen travelling a short distance and a low-level tail of long-distance distribution (Ellstrand 1992). The extent of the tail of the dispersal curve is important as long-distance pollen dispersal is often underestimated. Hybridisation between the native E. ovata and planted E. nitens in Tasmania has been documented on several occasions (Barbour et al. 2003, 2002). In a recent study the pollen dispersal curve was investigated; it showed an average of 7% hybrid seed within 100 m of the plantation boundary, dropping to 0.7% by 200–300 m. Hybrids were still detected 1.6 km from the boundary (Barbour et al. 2005). Pollen dispersal is directly related to the method of pollination dispersal, pollinator behaviour and

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Species name

Tamarix aphylla

Acacia nilotica

Mimosa pigra

Parkinsonia aculeata

Prosopis spp.

Salix spp.

Chrysanthemoides monilifera ssp. monilifera

Chrysanthemoides monilifera ssp. rotundata

Lantana camara

Ulex europaeus

Rubus fruticosus

Annona glabra

Tipuana tipu

Cytisus multiflorus

Common name

Athel pine

Prickly acacia

Mimosa

Parkinsonia

Mesquite

Willows

Boneseed

Bitou bush

Lantana

Gorse

Blackberry

Pond apple

Rosewood

White Spanish broom

50 000

Dune stabiliser, revegetation

140/fruit

80/fruit

6 million/ha

Ornamental

Shade, stock feed, 10 000 timber, ornamental

Rootstock for custard apple

Fruit

Ornamental

Escapee, ornamental

12 000

50 000

Dune stabiliser, ornamental

2.5–3

15

Explosive pods, water, animals, mud

Wind, water

Water, feral pigs, cassowaries

Birds, pest animals

Water, soil, ants, birds

3e 2–3

Birds, foxes

Wind, water, pest animals

Birds, pest animals, stock

1.5

5–7

6–7

100 000

Shade, fodder, ornamental

Water, mud

Water, animals, people

Stock, water

7–9b 4–6

Wind, water

1

Seed size (mm) Dispersal

10

220 000

175 000

500

000a

Seeds per plant

Shade, ornamental, 5000– hedge 13 000

Escaped from botanic gardens

Shade, fodder, ornamental

Shade, windbreak, erosion control

Reason for introduction to Australia

Yes

Insectg 3 years

Insect

Yes

Yes

Insect

Yes f

Insect

2 years

No

Insect

Insect, selfd

Insect

Insect, self

Insect

Insect

Insect, wind, selfd

Insectc

Insect, wind

Pollinator

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

No

Dormant

No

2 years

18 months

1 year

1–3 years

1–3 years

2–3 years

2–3 years

4–12 months

2–3 years

3 years

Time to flower

Yes

Yes

Yes

No

No

Yes

Yesd

No

No

Yes

Vegetative dispersal

Table 7.3: Biological attributes of the woody species identified as Australian Weeds of National Significance or as environmental alert weeds

118 Agroforestry for Natural Resource Management

Ornamental

Species name

Retama raetam

Acacia karroo

Acacia catechu

Koelreuteria elegans ssp. formosana

White weeping broom

Karroo thorn

Cutch tree

Chinese rain tree*

5

5–8 Water, birds

Water, mud, animals

g: Rodriquez-Riano et al. (2004)

f: Faithfull (2005)

e: Anon (2005a) <www.cdfa.ca.gov/phpps/ipc/weedinfo/ulex.htm>

d: Anon (2005b) Global invasive species database. <www.issg.org/database>

c: Stone et al. (1998)

b: Anon (2004a) Flora Zambesica <www.kew.org/efloras>

a: Frost (2005)

No

No

No

5–8b

19 000 Wind, water, animals

No

Vegetative dispersal

6.5

Seed size (mm) Dispersal

>1000

Seeds per plant

Information cited is from weed management guides (Anon 2003) unless specified otherwise.

Ornamental

Escaped from botanic gardens

Ornamental, escaped from botanic gardens

Reason for introduction to Australia

Common name

Time to flower

Yes

Yes

Yes

Yes

Dormant

Pollinator

7 – Environmental risk in agroforestry 119

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factors affecting pollinator movement. In general, wind-pollinated species show greater pollen dispersal distances than animal- or insect-pollinated species, but dispersal is highly idiosyncratic and can vary greatly in different environments at different times and even for different individual plants. Research is required on the major pollinators and factors affecting their foraging behaviour. Recent studies overseas have shown that fragmentation has led to increased gene flow among scattered trees and populations (Bacles et al. 2005; White et al. 2002). This is likely to be a factor in Australia: recent analysis of gene flow in fragmented populations of E. wandoo in the Western Australia wheatbelt revealed that 50% of the pollen load of some trees came from >1 km away (Byrne et al. 2008). In contrast to the E. nitens–E. ovata studies (Barbour et al. 2003, 2005, 2002) noted above, the E. wandoo data indicate a fat-tailed pollen dispersal curve with substantial levels of long-distance pollen dispersal. Long-term monitoring of introgression Data is required on the long-term level and impact of hybridisation and introgression in populations where hybridisation is known to occur. Research should determine whether introgression is more likely to confer neutral, advantageous or disadvantageous genes to native genotypes. Long-term introgression of crop genes into wild relatives has a history of producing or enhancing weediness traits in native populations, creating new weeds or enhancing the effects of existing weeds (Ellstrand et al. 1999). This may occur if key traits influencing weediness are transferred, such as the ability to propagate vegetatively, herbicide resistance or insect resistance. In order to judge the ecological impact of hybridisation or repeated introgression for given taxa, it is probably necessary to perform controlled crosses and test the hybrids under closely controlled field conditions (Tiedje et al. 1989). This is extremely difficult and time-consuming in agroforestry due to the long generation time of most tree species. Impacts on local biodiversity To assess risks posed to native populations, it is important to have an understanding of the occurrence, size and distributions of remnant native

populations as well as their genetic diversity. Placing a conservation value upon individual populations makes it possible to evaluate acceptable levels of genetic contamination. If a population has low conservation value (e.g. its representative species and genetic diversity is captured in several other populations and conservation reserves) and the likelihood and impact of genetic contamination are both low, the benefits from agroforestry planting may be considered greater than the risk.

Managing the risk Risk management framework

Risk management is a common and essential aspect of many industries, businesses and organisations. Agroforestry is no different. Environmental risk is one aspect that requires evaluation in a risk management framework. The first steps in risk management are to identify and evaluate the risks through use of assessment protocols. Once the risks are identified and evaluated, decisions can be made about the level of risk that is acceptable, in association with the level of management that is feasible. Where there is a high feasibility of managing risk, the information from the risk assessment is used to develop strategies for minimising the impacts and for design of management practices. Some general guidelines for the management of weed and genetic risk in agroforestry are presented here, although they will need to be assessed in specific situations for specific species. Weed risk management

Guidelines were developed for biodiversity conservation in softwood plantations (Pinus radiata) in 2000, incorporating the results of an extensive research project at Tumut, New South Wales (Lindenmayer 2000a). Some of the key recommendations in relation to weed and genetic risk include:

s s s

further develop and use reproductively sterile cultivars for plantation establishment; remove existing wildlings from remnant vegetation patches; develop hygiene protocols for machinery to reduce the spread of other weeds, e.g. blackberry.


Virtue and Melland (2003) provide options for managing species that have a high weed risk and a high utility, such as Pinus and Acacia species, though further research into many of the options is required. The options include:

s s s s

setting minimum planting distances from native vegetation; introducing measures to limit seed production and dispersal; ensuring routine monitoring and control of escapees; choosing less weedy species/cultivars.

Some agricultural industries are including management options similar to those suggested above, in voluntary codes of practice. An example is the code of practice adopted by olive growers in South Australia. Some of the code’s requirements include:

s s s s s

a buffer zone of 200 m within property boundaries between orchards and the nearest significant native vegetation; if there are no trees in the buffer zone, a bird perch must be provided every 100 m around the perimeter, as birds are major dispersers of the seed; removal of fruit from the trees each year, including spilt fruit on the ground; active fox control and netting or scaring of birds to reduce seed dispersal; an olive-free zone around commercial orchards of 20–50 m to allow monitoring and access in case of fire.

Genetic risk management

Managing genetic risk through cultural management practices, including processes such as planting so that flowering times do not coincide, or harvesting before flowering, are virtually impossible to implement effectively with the long lifecycles of agroforestry species. The most reliable practices to limit gene flow include keeping the agroforestry crop isolated from native populations by distance or the use of barriers. Isolation distances Paternity studies have enabled estimates of gene flow distances to be made for a number of species. In crop plants, specific minimal isolation distances

have been suggested at 300 m for self-fertilising species and 800 m for primary outcrossing species to maintain varietal purity, despite these distances allowing for contamination of up to 10% (Raybould and Gray 1994). However, levels of pollenmediated gene flow in natural populations and crop/weed complexes has proven to be much more extensive than initially predicted (Ellstrand 2003; Raybould and Gray 1994) and these recommendations may be inadequate. Direct studies of gene flow in agroforestry species in Australia are limited. In eucalypts, a low level of long-distance pollen dispersal has been inferred from isolated mature hybrids found in native stands up to 1 km from a boundary (Potts et al. 2001) and long-distance pollen dispersal of around 5 km has been reported although not verified (Potts 1990). Pollen dispersal from a planted stand of E. loxophleba ssp. lissophloia into remnant populations of ssp. supralaevis occurred over 2 km (Sampson and Byrne 2008). Similarly, in A. saligna pollen dispersal from planted stands into remnant populations over distances of 1.5 km has also been documented (Miller et al. 2007; Millar and Byrne 2007). Until we have greater knowledge of what influences pollen dispersal curves in native species, general guidelines include:

s s s s

isolation distances should be greater for windand bird-pollinated species than for insect-pollinated species; isolation distances should be greater for a patchy than for a spatially continuous flowering resource; at least 1 km is required for pollen immigration to be effectively reduced in a continuous native forest abutting a plantation with synchronous flowering (Potts et al. 2001); estimates of gene flow are a guide and specific estimates will always be imprecise due to sensitivity to environmental conditions and the spatial distribution of source and sink populations.

Border rows Barrier rows, or guard rows, of different noninterbreeding species of the same species may be incorporated around agroforestry plantations to intercept a portion of pollen entering or leaving the crop. For insect-pollinated eucalypts, 100–200 m

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buffer zones of non-interbreeding trees have been suggested to protect seed orchards from contamination by pollen from natural populations (Potts et al. 2001). In crop species, models have predicted that buffer zones will substantially reduce pollen leakage from plantings (Manasse and Kareiva 1991). The effectiveness of this strategy and the distances involved need to be investigated in agroforestry systems.

Conclusion There are weed and genetic risks associated with the use of native species for large-scale revegetation for agroforestry, even within their natural range. This does not mean that the use of native species should be abandoned – they still have considerable advantages over exotic species. It does indicate that the use of native species should be developed within a risk management framework (Potts et al. 2003). Risks must be identified, adequate information obtained to assess the level of risk and evaluations of management options made. Weed risk assessment is relatively advanced and readily evaluated. Many genetic and ecological factors interact, making it difficult to assess the impacts of genetic contamination from hybridisation. However, a greater understanding of the dynamics of gene flow will allow some assessment of the risks involved in using particular taxa in large-scale revegetation for agroforestry. Risk assessment should be an integral part of environmental impact assessments of agroforestry plantations. These risk assessments will contribute to the development of informed decision-making processes in the implementation of agroforestry revegetation systems and ultimately aid in the development of land uses that protect and enhance biodiversity in the agricultural landscapes of southern Australia.

References Adair RJ (1995) The threat of environmental weeds to biodiversity in Australia: a search for solutions. In Conserving Biodiversity: Threats and Solutions. (Eds RA Bradstock, TD Auld, DA Keith, RT Kingsford, D Lunney and DP Sivertsen) pp. 184–201. Surrey Beatty & Sons, Sydney.

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8 Landscape aesthetics and agroforestry Grant Revell

Introduction All landscapes are essentially social constructs – they are sensed environments that tell us a lot about how we see ourselves as casual observers, concerned environmentalists or active land managers in the Australian setting. Landscapes help us understand where we have come from and where we are heading as individuals or as communities, sometimes with (or without) any detailed knowledge of the natural or cultural environment. Greider and Garkovich (1994) suggest that: ‘Landscapes’ are the symbolic environments created by human acts of conferring meaning to nature and the environment, of giving the environment definition and form from a particular angle of vision and through a special filter of values and beliefs. Every landscape is a symbolic environment. These landscapes reflect our self definitions that are grounded in culture (Greider and Garkovich 1994, p. 1). Landscapes consequently not only conceptualise and represent our lives, they actually become our lives. As natural resource managers, we have begun to open up our eyes, hearts and minds to the ideas that such environmental resources have deeper social values attached to them – from very small private localised holdings to very large industrial or regionally based landscapes and their communities. Knowing the cultural meanings attached to these social landscape values can often help land managers realise the complexities of environmen-

tal management practices. In turn, this shift in thinking necessitates a more innovative and creative response to agroforestry land management that embodies the integration of a range of social and physical sciences and arts disciplines and professions, their knowledge sets and associated practices (Winchcombe and Revell 2004). Today, the natural resource management of agrarian landscapes has every opportunity to value-add, as the cultural products of custodial care and stewardship of land and community are very much process-driven. It is this rich integrated management process that can support and set up the creative abilities for scientists and artists to collaborate, ensuring that agroforestry decision-making considers the potentially negative impacts on social, biophysical and economic landscapes, while optimising the positive. Indeed, the aesthetic values placed on agroforestry landscapes can be read as complex cultural outcomes or as barometers of good or bad integrated land management through the processes of land design. Consequently, natural resource managers are fast becoming the new aesthetic facilitators for holistic cultural resource management of our environments; as Greider and Garkovich (1994) might infer, ‘the creative managers of cultural symbols’. This chapter provides a detailed background to the relevant theories and practices of landscape aesthetic assessment and an outline of the typically integrated landscape design requirements of agroforestry systems. From the outset, it should be 127

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noted that this study of agroforestry systems is not well-established in Australia. The body of related literature is small, is often too generic and is historically represented from research and practice undertaken primarily in the UK and North America. This chapter complements the locally documented work of Van Pelt (1980), Ramsay and Paraskevopoulos (1993), Revell (1997), Cleary (1999), Cowan and Revell (2000) and Winchcombe and Revell (2004). It provides a useful beginning for all students and practitioners interested in developing stronger design sensibilities for the agroforestry management of specific and meaningful places in Australia. It asks questions. How do we make environmental features important for biological reasons, and economic values important for landscape aesthetic reasons? Is this new integration of values through the creative thinking and practices of agroforestry simply a new land aesthetic?

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The following key terms are used (adapted from Winchcombe and Revell 2004).

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Aesthetic landscape values are the experiential responses derived from an appreciation of a complex mosaic of environmental elements. They include natural and cultural attributes with visual and non-visual aspects such as sound, smell, sense of place, emotional response and all factors having an influence on human attitude (O’Brien and Ramsay 1992). These aesthetic values are derived from a structural inquiry into any act of representation and how it is constituted by ideologies of nature and culture, as opposed to connoisseurship, taste or style (Weller 1997). Analysis is the process by which the landscape is broken down into components and understood in terms of its particular compositional elements and behaviours (Revell and Cleary 1998). Assessment is a process of synthesis. It is the expression of composite value based on the value of individual landscape components (Revell and Cleary 1998). Evaluation is the process where landscape assessment results are examined and used to

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make decisions about alternative landscape futures (Revell and Cleary 1998). Inventory refers to the identification and collection of landscape data. Inventory is without value judgements (Revell and Cleary 1998). Landscape refers to a place or defined area of land exhibiting aesthetic, historic, scientific and social values (Stuart-Street and Revell 1994). Landscape architecture is the development of a harmonious, sustainable and enriching fit between human systems and natural systems. Landscape architecture is art and science, both analysis and intervention (Riley 1998). Landscape design (as a verb) refers to the creative processes that prepares strategies or plans for the spatial and meaningful fit or intervention between these biophysical and social systems. Landscape character is the combination of natural and cultural landscape characteristics which allow people to differentiate between one place and another. Landscape value is generally derived from a process of valuation and can be expressed numerically (Litton 1979). Participatory design typically involves a close working relationship between the designer, the client and other members of a community interested in a project, including other scientists. Joint contributions are often made by all parties throughout the design process. It is very different from design consultation, which often reinforces the designer as the expert and ignores the design interests and talents of the landowner, manager and wider community. The educational and learning outcomes for all parties is often restricted in design consultation models. Scenic quality is the relative visual character of a landscape, expressed as an overall visual impression or value held by society after perceiving a segment of land/water (Stuart-Street and Revell 1994). Sense of place describes a holistic experience which defines a person’s perception of place and their relationship with it. Visual landscape is that portion of the landscape within a person’s view (Stuart-Street and Revell 1994).

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Visual landscape management is the act of managing the visual landscape, its environmental values, resources and attributes.

Landscape assessment models for agroforestry As discussed and taken from Winchcombe and Revell (2004), the actual assessment of landscape aesthetic values is often carried out as part of the site inventory and analysis procedures typically employed by landscape planners and landscape architects in land design or management projects. There are few agroforestry planning and design projects in Australia that have consciously and systematically employed these environmental assessment procedures or techniques (Winchcombe 2000). The work of Daniel and Vinning (1983) and Lamb (1993) describes a number of key theoretical models or conceptual approaches employed in assessing landscape aesthetic values, as follows:

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component model; formal aesthetic model; ecological model; psychophysical model; psychological model; phenomenological/experiential model.

The component model and the formal aesthetic model assume that aesthetic value is intrinsic to the physical attributes of the landscape. These approaches are considered to be descriptive inventory approaches. The formal aesthetic model is influenced by picturesque perceptions of the landscape, not unlike those that influenced the development of the landscape painting schools of the 18th and 19th centuries. Visual qualities such as form, line, colour, texture and scale are factors contributing to landscape quality. The ecological model recognises that much of the concern for landscape quality or amenity value stems from a more general concern for the satisfactory management of the natural environment, where the ideas of naturalness are the significant evaluative factors. Natural resource managers and landscape architects work closely with expert biologists and ecologists to determine the relationships between ecological and aesthetic values of the environment. Ecological units, classes or functions (e.g.

riparian zones, edges, open woodlands, closed forests) are often classified for their significance and disturbance, and corresponding aesthetic evaluations are made. Rare and endangered landscapes are often given high aesthetic values, and humaninfluenced or modified environments are judged as low. The closer a development (or agroforestry system) simulates or mimics a natural environment, the more aesthetically acceptable it becomes (Daniel and Vinning 1983). The psychophysical model assumes that the physical attributes of the landscape determine the psychological response of the viewer. When developing management guidelines and management strategies for forests and agroforestry systems under their control, the Forests Commission of Victoria and the Department of Conservation and Land Management in Western Australia employ this approach. The psychological model is more concerned with people’s experience of the landscape. It explores the reasons behind a viewer’s response and includes things such as memory, past experience, interests and cultural background. Ultimately it gives more weight to landscape user preferences than do the other approaches. It has been criticised from a problem-solving point of view for lacking an applied orientation or clear practical outcomes. It is considered as a hypothesis builder rather than an analysis type model. Nonetheless, it can give explicit knowledge about how certain landscapes are valued and how they are managed, with or without agroforestry systems. The phenomenological/experiential model approaches landscape from the viewer’s deeper experiences, feelings about and expectations of the environments that surround them. Through a systematic content analysis, common experiences are identified and used as a basis for a deeper understanding of the perceived landscape. Landscape quality is considered a very intimate interaction between the observer and the environment. It is influenced by a whole set of complex cultural factors, including the histories of the environments as well as the observers themselves – their cultural backgrounds, education, intentions and motivations. An understanding of landscape quality is derived from the meanings and significance of its environmental attributes and associations (Daniel and Vinning 1983). When landscape architects and

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Figure 8.1: Landscape aesthetic value assessment process (adapted from O’Brien and Ramsay 1992).

natural resource managers attempt to define or describe the sense of a place, they often look at the potential reawakening of innate senses and abilities to understand the aesthetic phenomena of human behaviour and environment relations. Each model has strengths and weaknesses arising from the critical elements used to describe a landscape’s quality, the objectivity of the evaluator and the repeatability of the result. They are included to illustrate the typical methods employed in assessing landscape attributes for their qualities and values, and to provide the background for a potential approach for agroforestry management systems (Winchcombe and Revell 2004). The realities of choosing the ‘best’ landscape assessment model largely depend on the type of the agroforestry project, in particular the brief set by the land manager or property owner. Design expectations are also influenced by various legislative requirements and/or government codes that require inter-

pretation in the greater property planning and design process. An example of a best practice landscape assessment model is the methodology developed by the Australian Heritage Commission (Figure 8.1). It is used to assess the aesthetic value of landscapes for inclusion into the National Register, according to certain criteria. It is a highly complex and comprehensive model for assessing landscape aesthetic values, developed as an amalgamation of the models previously described by Daniel and Vinning, and Lamb. One of its key features is acknowledging the inclusion and integration of other environmental assessment methodologies, e.g. for the determination of social, historical or scientific values associated with particular environments. In applying this type of assessment model, the extra knowledge and understanding of what influences the construction of landscapes from environments and their attributes can be significant when shared


among land managers and other specialists involved in the planning and design of agroforestry systems (Winchcombe and Revell 2004).

Landscape management planning and design guidelines This section discusses the major visual landscape design principles and guidelines applicable to agroforestry systems (Revell 1997, pp. 74–79). Admittedly, designing agroforestry systems predominantly on scenic or visual quality grounds is essentially a reductionist approach to managing landscape values, as explained above. However, it offers a point of departure for the landscape architect or land designer working with other pragmatic specialists in the arena of agroforestry. In that 1997 study I asked (and attempted to answer) two significant questions regarding the actual landscape planning and design of agroforestry systems. 1

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What are the major visual design principles and guidelines to consider in the layout of agroforestry landscapes? What are the essential steps in the scenic quality?

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In response to the first question, the following major visual design principles and guidelines were proposed. Design principles and guidelines for background-scale landscapes

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Agroforestry plantation scale should reflect the scale of the surrounding landscape. For example, large open valley landscapes can accommodate a greater area of plantation establishment and harvest area than can smaller ones. Impacts can be minimised by separating plantation areas with existing vegetation or by creating cells of varying age classes. Patterns of the plantation areas should reflect or imitate surrounding land use patterns. Plantation design should, where possible, follow existing landscape lines such as tree line, road line, fence line, creek line and ridge line. The delineation of the plantation establishment or harvest area should respect these. Avoid reinforcing lines if they are incongruous with the surrounding landscape. For example, in a land-

scape setting which predominantly exhibits free-flowing lines, avoid breaking skylines and reinforcing property and fence lines that are geometric in nature. Plantation access roads and firebreaks should be of low visual impact, preferably screened, with alignments following contours, existing road patterns, vegetation lines etc. rather than artificial property boundaries. In visually sensitive areas, potential plantation impacts can be reduced by enhancing and extending existing vegetation areas with similar species plantings. These plantings could be protected from future harvesting, thus optimising wildlife, soil, water and recreation values. If harvested, the sequence and timing of cut should be separate from the main plantation harvest schedule.

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At this scale middleground areas dominate the landscape. The local appreciation of ridges, valleys and plains is offered. Unlike backgroundscale plantation areas, the whole plantation is unlikely to be viewed at one time. Perception of detail increases, colour and texture replaces shape, and pattern and line become the major visual or scenic elements. The outline of the plantation area should be defined by gullies, spurs and ridges, and should borrow from the lines offered by the surrounding landscape. Plantation areas should be defined as individual units broken up by ridges, drainage lines and dominant land use patterns. Avoid over-reinforcing areas of maximum visual contrast such as tree lines, skylines, vegetation changes etc. Ensure that such contrasts reflect other contrasts (if present) in the surrounding landscape. For example, in natural settings, edges of vegetation change can be softened by sympathetic boundary lines, gradual change in density or age class across the interface, or the use of species of different form, colour and texture. For skyline edges, maintain ridges with species typically dominant within the surrounding landscape. For example, avoid pines in a hardwood forest setting. If pines are necessary

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then locate them below the skyline and vary age class and planting density where possible. The harvesting of ridge or skyline plantation areas should be sequenced to reduce the extent of clearing disturbance visible at any one time, or to enable adjoining areas to be successfully regenerated. The upper margins of any planted area are prominent components of the planting design. In settings of a strong natural character these margins should rise or point up in the valleys and depressions, and fall or point down on the spurs. The upper margin should be located so that any open ground above the planting area is of sufficient size to reflect the scale of the hill cap, knoll or ridge. The visual impacts of power lines, transmission towers and corridors can be reduced by creating a series of irregular planting spaces. Trees can be planted closer to power lines opposite pylons or towers than in mid-span, and smaller trees and shrubs can be grown closer still.

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Design principles and guidelines for foreground-scale landscapes

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At this scale foreground areas dominate the landscape. All perception of the background and middleground landscapes is lost. The microscale dominates, with occasional glimpses extending to the middle and background areas. The observer is virtually in the plantation or revegetated landscape. There is total perception of details of individual trees, their colours and textures, their diversity or uniformity. Visual change to the plantation is most easily detected at this scale. These plantation landscapes require a high degree of scenic quality management throughout all stages of the plantation program. Follow the visual expression of the surrounding foreground landscape. Avoid contrasts to these details. For example, in a uniform, patchwork, geometric, foreground agricultural landscape setting, the visual character of an exotic, regimented-looking blue gum or pine tree belt could enhance the local landscape. In natural non-uniform settings encourage diversity through the physical separation of plantation sections or compartments. These

areas may differ in age, species mix, planting density, thinning regime etc. Maintaining visual penetration through the plantation can enhance visual quality of the plantation landscape. This can be achieved by an open or clumped planting density or through thinning techniques. In natural-appearing landscape settings, thinning regimes should be non-uniform. Conversely, in geometric or culturally dominated landscapes thinning regimes should be regular and uniform. Access tracks and firebreaks should be designed and constructed with low visual impact. In sensitive areas, avoid visual impacts created by plantation debris, slashed material etc. Reduce impacts by screening, burning, mulching or scattering debris away from seen areas. In visually sensitive areas, harvest areas should be of minimal size in relation to the overall plantation landscape. Felled areas should not dominate over unfelled areas. In visually sensitive areas, employ harvest sequencing techniques. Near-roadside vegetation, for example, can reduce the visual impacts of adjoining harvest areas. Final harvesting of near-roadside vegetation should take place after surrounding coupe areas have been regenerated or replanted, and grown to become a strong enough visual element. Nearroadside vegetation could also be treated as multi-aged stands, and harvested sequentially within themselves.

In response to the second question, the essential steps in the scenic quality design of agroforestry landscapes, there are important steps in the scenic quality design process.

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Observe the agroforestry landscape setting. Take time to see and appreciate the visual character of the surrounding landscape. What are the local, district or regional scenic attributes or features of these landscapes? Map these features on suitably scaled topographic maps and aerial photographs. Try to identify any patterns in land use, land form, hydrography or vegetation cover. Consider whether the proposed tree planting will maintain, enhance or have a negative visual impact on natural or cultural landscape features?


Table 8.1: Comparative analysis of integrating landscape aesthetic quality management objectives with other agroforestry system objectives Other agroforestry objective

Impact of other objective on landscape aesthetic quality management

Impact of landscape aesthetic quality management on other objective

Nature conservation

Positive impact of conserving or reconstructing a natural aesthetic on the farm and in the catchment. Maintaining remnant vegetation cover will substantially benefit landscape quality.

Ensure landscape aesthetic quality revegetation objectives increase nature conservation benefits to farm and catchment.

Soil conservation

Design landscape quality management to General increase in landscape quality, increase all conservation benefits. Use especially with vegetation along valleys, local plant species where possible. ridge lines and stream lines and around water bodies and discharge areas. Impacts of poor soil conservation management often result in low scenic quality. Avoid stock access to stream lines and dams where possible.

Salinity control

Revegetation for salinity control generally increases landscape quality depending on species type and planting layout. Poor tree growth and potential death will reduce scenic quality.

Design salinity control revegetation measures to enhance the visual character of the farm and catchment landscapes.

Shelter

Positive impact if vegetation shelter layout follows natural and/or cultural landscape characteristics. Species diversity (grasses, shrubs and trees) will often increase landscape aesthetic quality.

Shelter may be compromised depending on wind direction and land form patterns. Ensure remnant vegetation is retained, protected and managed for additional stock shelter where required.

Fodder

Positive impact on landscape aesthetic quality depending on species type and design layout. For example, geometric shaped, exotic and/or heavily grazed vegetation plots can be visually obtrusive in the landscape, especially in foreground areas.

Minimising visual impacts from grazing and species type may be unacceptable, depending on visual landscape character of the farm and surrounding areas.

Windbreaks

General benefit to landscape aesthetic quality depending on scale, shape, orientation and species type of the windbreak.

Notwithstanding soil conservation requirements, orientation of revegetation layout may compromise windbreak benefits. Maximise use of remnant vegetation.

Timber production

General benefit depending on species type and plot layout. Visual impacts of conventional silvicultural prescriptions and harvest operations usually lower landscape aesthetic quality.

Landscape quality management may require alternative and creative silvicultural and harvesting practices. Need not affect economic output.

Agroforestry tourism

General benefit to landscape aesthetic quality and environmental education experience.

Landscape aesthetic quality management should enhance the agroforestry-based tourism experience. Should increase economic output.

Sense of place

Agroforestry systems encouraging a greater sense of place and perceived belonging to the farm locality usually increase landscape aesthetic quality.

Scenic quality improvements through agroforestry systems can be designed to enhance local and regional landscape character and identity, general sense of place, and liveability within the agrarian environment and surrounding community.

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How will the planting area look in five, 15, or 30 years? What impacts will the plantation have on major private or public views and vistas of the landscape, during all establishment and harvest periods? Are there local community landscape design ideas and aspirations that should be respected? How can the planting area be planned and designed to accommodate such scenic quality considerations? Refer to the guidelines and principles above. Prepare preliminary planting design plans, visual simulations and prescriptions to suit integrated agroforestry management objectives and share these with neighbouring property owners/residents. In scenically sensitive areas, maintain a landscape evaluation and monitoring program. Take periodic photographs and monitor public and private feedback.

New projects for integrated agroforestry lead land management In recent years the Western Australian Department of Land Management and the University of Western Australia, along with a number of noted private projects, have embarked upon a series of experimental integrated agroforestry land management projects (Winchcombe 2000). These projects have brought together a range of land management specialists representing particular scientific fields and associated practices. The landscape architects, natural resource managers and property owners have represented many of the arts-related disciplines and have essentially become the designer-integraters or design-facilitators of the more holistic land management practices demanded in these innovative projects. There is much to say about these projects; they have successfully achieved a set of integrated and pragmatic working plans that have delivered an agreed resource management consen-

Further guidelines for landscape quality management

New guidelines will need to be developed, as greater resources for integrated agroforestry landscape perception research become available. Research specialists should collaborate on studies that seek to understand potential connections between aesthetics (landscape quality), economics and ecology, then develop their own hybrid landscape assessment approaches to assist agroforestryrelated projects (Winchcombe and Revell 2004). Table 8.1 considers the value of landscape quality management (simple integrated landscape planning and design initiatives) in an attempt to quantify the potential relationships and impacts on other land management objectives applicable to integrated agroforestry systems. For some, it should dismiss the negative connatations concerning the worth of landscape aesthetic management if treated as a non-integrated resource management tool. As noted, further integrated research is required to test these potential management impacts and interrelationships across the greater agroforestry landscape. This challenge should be taken up by agroforestry-related scientists and artists whether or not they are directly affected; there are multiple benefits on offer from these new designed landscapes.

Figure 8.2: Specialists gather to develop an integrated and participatory agroforestry property plan (photos by Grant Revell).


Figure 8.3: The agroforestry landscape design process (adapted from Winchcombe and Revell 2004).

sus from all those around the planning and design table (Figure 8.2). From a landscape management point of view, in these projects the landscape architects and natural resource managers have guided a rich design process that attempted to quantify and represent, on scaled site plans and aerial photographs, a series of conventional planning and design tasks, namely resource and value inventories, spatial analyses of that data, site design opportunities and constraints, optional agroforestry design concepts, preferred concepts, agroforestry design implementation plans and design evaluation methods. Figure 8.3 illustrates this integrated landscape design process. Figures 8.4a and 8.4b show innovative yet simple computer-generated digital terrain modelling exercises used to quantify, evaluate and redesign a significant view-shed in an agroforestry property. The landowner wanted to protect and enhance the visual character of the paddock sky lines where the older tree species of marri (Eucalyptus calophylla) dominated. The proposed pine and eucalyptus tree belts were designed and located so as to avoid any major interruption to these important endemic

views and associated landscape amenity. Where possible, the imposed lines created by the new tree belts visually complement the remnant vegetation lines and patterns. Proposed gaps between tree belts were designed to mimic the scale of gaps found in surrounding healthy remnant vegetation patches. Figure 8.5 shows a hand-drawn sketch plan developed for another key view-shed, on a separate project. The design process is assisted by realistic site photography and design intentions are annotated on the sketch for communication with the planning team. The visual landscape attributes of scale, pattern, form, line and contrast within the paddock’s composition and setting, for example, are simply manipulated in the design process – trees are used to mimic the local landscape character or to enhance it, not unlike composing an aesthetically pleasing landscape painting. This time, the design process also aimed to improve the paddock’s ecological well-being. Both these design techniques are relatively quick, easy and cheap to employ on-site. Most importantly, they are iterative design devices and

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NOTLEY’S “WESTERN NODES” FARM - DANDARAGAN EXISTING VIEW OF WESTERN RIDGE

Remnant vegetation

NOTLEY’S “WESTERN NODES” FARM - DANDARAGAN PROPOSED VIEW OF WESTERN RIDGELINE

Remnant vegetation Eucalypt shelterbelts Pine shelterbelts

Figure 8.4: Computer simulations of (a) existing and (b) proposed design conditions in the protection and enhancement of a significant property view-shed. (c) Tree rip-lines prepared and located in accordance with the plan in Figure 8.4b. (Images by Grant Revell and Phil Durrell).


Figure 8.5: Hand-drawn sketch of proposed agroforestry modifications to a key property view-shed (image by Grant Revell).

allow the agroforestry planning team to evaluate the proposed results and make quick decisive modifications to the concept and implementation plans. Figure 8.6 illustrates a final agroforestry master plan for a property named Brooklyn, in the township of Bridgetown, south-west Western Australia (Cowan and Revell 2000). Its development employed the design process indicated in Figure 8.3, and gives the land manager a strong visual outcome of the integrated planning work. A formal aesthetic and ecological landscape assessment model was used to identify the aesthetic landscape qualities of the property. The master plan represents a spatial approach to systematically undertaking resource and value inventories, identifying clear design opportunities and constraints, optional and preferred design concepts and implementation measures. Most importantly, it remains a flexible document – a strong conversation piece in the further evaluation of its likely performance in the landscape. It adapts itself as a decision-making tool to a staged implementation schedule, influenced by the availability of other resources and knowledge sets.

The land design criteria developed by the Brooklyn farming family and the project landscape architects are set out below. Landscape character

The aims were to:

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preserve and strengthen the existing 19th-century picturesque nature of the property’s open space and parkland character. Where possible, different tree species were used to reflect the ecological and cultural landscape diversity across the property; maintain and enhance vegetated ridge and skyline characteristics of the Bridgetown district landscapes; ensure key private and public views are maintained, enhancing visual prospect and visual refuge; shape, taper and/or feather agroforestry plantation edges to complement strong land forms. Where possible, the aesthetic principles of shaping the belts according to the contours are followed, with tapering belts up valleys and down spurs;

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LEGEND Fence – Existing Fence – Removed Fence – New Dam Riparian Vegetation Remnant Vegetation Wood lot Shelter Belt ‘Agro-cology’ House / Shed Domestic Precinct Contours (10m) Visual Link Ecological Link Drainage / Flow

Figure 8.6: Integrated agroforestry design master plan (Cowan and Revell 2000).


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focus on and strengthen the strong cultural character of the homestead, its formal gardens and the nearby old school area; where possible, enhance the farm-stay facility views and associated landscape experiences.

Landscape ecology

The aims were to:

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fence and revegetate riparian zones; maximise water uptake in discharge areas and mitigate further salinity expression by revegetating ridges and midslopes identified by the project hydrogeographer; fence and link remnant vegetation, and connect to surrounding plantation areas to create continuous wildlife corridors; interpret the property’s ecological health to help benefit farm-stay objectives.

Stock management

The aims were to:

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ensure visual access across and into paddocks for ease of stock management; orient agroforestry layouts as windbreaks to minimise wind exposure; where possible, fence to soil types for the purpose of controlled grazing.

The master plan (Figure 8.6) illustrates the potential for the whole farm landscape to be envisaged as a rural garden or utilitarian arboretum. Landscape rooms were identified according to their preferred agrarian character and use, and designed accordingly. Integrated agroforestry systems have a design potential, albeit in a less formal manner, similar to the traditions of 17th- and 18th-century English estates and French chateaus where integrated agricultural, horticultural, forestry, recreational, artistic and residential pursuits within the designed landscape were commonplace. These traditions followed the idea that the countryside, with all its appeal of nature, should be laid out for greater public view and enjoyment. The shared sense of place and the formal aesthetic qualities of landscape were based on a public ideal not unlike the values promoted by the picturesque landscape painting schools of the time. Prolific English designer and writer Stephen Switzer’s publication

in 1715, for example, entitled The Nobleman, Gentleman, and Gardener’s Recreation (reissued in 1718 as Ichnographia Rustica), gave explicit advice on the design of large-scale forest and rural garden landscapes (Jellicoe et al. 1986). Switzer’s agroforestry design expertise is shown in Figure 8.7, where the forest garden is laid out on a massive scale incorporating a carefully planned framework of tree belts, avenues and plantations defining other agrarian rooms, smaller gardens and open-space areas, all laced together with a series of hierarchical walkways, carriageways and vistas. This may well be one of England’s earliest attempts at designing public agroforestry as an aesthetic and utilitarian practice of rural gardening.

Conclusion If natural resource managers are to become the new creative managers of meaningful cultural symbols in our environments, then as a discipline and profession we need to develop innovative ways to collaboratively design agroforestry systems to suit a complex array of societal values. Custodial relationships with the land need to embrace a diversity of economic, biophysical, social, spiritual and religious attachments to the great environmental nexus of nature and culture. The ecological management of rural and urban lands, for example, may develop a nature and human relationship, aesthetic or amenity that mimics natural systems that existed in some historical context. More importantly, it could encourage a whole range of additional and exciting cultural expressions and understandings of nature. It is these designed, contemporary narratives of integrated land management that offer so much in the appreciation and definition of who we were, who we are, and who we hope to be. We have vast opportunities to learn about Australian nature and culture through an honest and genuine design and management of its environment. This is the practice of both scientific and artistic intelligence. Economics will probably dictate these greater agroforestry design opportunities, and the more visible or public lands will embrace stronger aesthetic design care, stewardship and return – not unlike those designed forests and rural gardens of the 18th century. The desirable character of the land

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Figure 8.7: Stephen Switzer’s agroforestry plan of a forest or rural garden, from Ichnographia Rustica (1742 edn, vol. 3, reproduced in Jellicoe et al. 1986) (image courtesy of Oxford University Press).

will be protected or enhanced, land prices may rise and other new economies like tourism, forestry or housing may benefit. For other custodians, it will require a careful balance of limited resources. Land design must be based on economics so that they can stay in place on the land, to maintain their deeper ties and healthy relationships to their specific environments and interconnected communities. Much has been discussed in recent landscape literature about the importance of cultures developing peaceful ecological aesthetics to land and its management (Nassauer 1997; Johnson and Hill 2002; Judith Wright in Tredinnick 2004). Some cultures

always had a strong alignment between aesthetics and ecology – somewhere along the way it just needed additional nurturing or resuscitation. Others have learnt not to see – and for the time being they may remain that way. Most importantly, as a discipline of land managers we are now realising that the ecological health of our environments, good or bad, demand revitalised aesthetic appreciation – a stronger alignment between an ecology of place and its corresponding aesthetics of landscape. The ‘beautiful’ picturesque aesthetic of land should perhaps no longer dominate our environmental values and landscape management. Open


parkland and tidy rural landscapes may start to disappear in favour of more truthful, designed, boggy and intermittent wetlands, biologically rich scrappy bushlands or brutally honest, industrialscale watertable pumping agroforestry plantations. We have begun to demand landscapes and economies that are both scenically and ecologically beautiful and perhaps, for the time being, ugly. Our aesthetic senses are changing, and their abilities to experience a deeper genius loci of specific environments will encourage a temporal appreciation of this strange ugliness. In time, a richer ecological aesthetic of landscape beauty or appreciation will arise, encouraged by the development or resuscitation of stronger senses for the qualities and systems of vernacular place. Finally, agroforestry systems and their designers need to play a vital role in the further research and application of some of the creative ideas discussed here. The experimentation, representation and interpretation of such ideas may help contribute to the cultural transformation of a healthier, more meaningful and sustainable Australian landscape. At the very least, we may see some more interesting integrated agroforestry systems.

References Abel N, Baxter J, Campbell A, Cleugh H, Fargher J, Lambeck R, Prinsley R, Prosser M, Reid R, Revell G, Schmidt C, Stirzaker R and Thorburn P (1997) Design Principles for Farm Forestry: A Guide to Assist Farmers to Decide Where to PlantTtrees. Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. Cleary J (1999a) Landscape and visual resource analysis. In Coastal Planning and Management. (Eds R Kay and J Alder) pp. 213–225. E & FN Spon, London. Cleary J (1999b) Shark Bay World Heritage Landscape Study. Department of Conservation and Land Management, Perth, WA. Cobham Resource Consultants (1993) Landscape Assessment Guidance. Prepared for the Countryside Commission, UK. Cowan L and Revell G (2000) ‘South-west farm land planning and design projects’. Unpublished report prepared for the Department of Conser-

vation and Land Management, Busselton Farm Forestry Unit. University of Western Australia. Curtis A and Race D (1996) A Review of Socio-economic Factors Affecting Regional Farm Forestry Development in Australia. Johnstone Centre, Charles Sturt University, Albury, NSW. Daniel TC and Vinning J (1983) Methodological issues in the assessment of landscape quality. In Behaviour and the Natural Environment. Advances in Theory and Research. (Eds I Altman and JF Wohlwill) vol. 6, pp. 39–84. Plenum Press, New York. Forestry Commission Tasmania (nd) A Manual for Forest Landscape Management. Government of Tasmania, Hobart. Greider T and Garkovich L (1994) Landscapes: the social construction of nature and the environment. Rural Sociology 59(1), 1–24. Itami RM (1993) Characteristics of landscape aesthetic value and implications for assessment methodology. In More Than Meets the Eye. Identifying and Assessing Aesthetic Value. (Eds J Ramsay and J Paraskevopoulos) pp. 13–21. Australian Heritage Commission, Technical Workshop Series no. 7. Jellicoe J, Jellicoe S, Goode P and Lancaster M (Eds) (1986) The Oxford Companion to Gardens. Oxford University Press, New York. Johnson BR and Hill K (Eds) (2002) Ecology and Design: Frameworks for Learning. Island Press, Washington DC. Kay R and Alder J (1999) Coastal Planning and Management. E & FN Spon, London. Lamb R (1993) Advancing arguments for the conservation of valued landscapes. In More Than Meets the Eye. Identifying and Assessing Aesthetic Value. (Eds J Ramsay and J Paraskevopoulos) pp. 23–36. Australian Heritage Commission, Technical Workshop Series no. 7. Leonard M and Hammond R (1984) Landscape Character Types of Victoria, with Frames of Reference for Scenic Quality Assessment. Forests Commission of Victoria. Leonard L, Delaney J and Revell G (2000) ‘Adaptation of computer software to evaluate the visual impact of farm forestry layouts’. Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. Project no. CAL-2A.

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Litton RB (1979) Descriptive approaches to landscape analysis. In Proceedings of Our National Landscape, a Conference on Applied Techniques for Analysis and Management of the Visual Resource. PSW Forest and Range Experiment Station, USDA Forest Service, Berkeley, California. Lucas OWRL (1991) The Design of Forest Landscapes. Oxford University Press. Nassauer JI (Ed.) (1997) Placing Nature: Culture and Landscape Ecology. Island Press, Washington DC. O’Brien M and Ramsay J (1992) ‘Assessing aesthetic values of landscapes for the Register of the National Estate’. Australian Heritage Commission, Canberra. Discussion Paper. Ramsay J and Paraskevopoulos J (Eds) (1993) More than Meets the Eye: Identifying and Assessing Aesthetic Value. Australian Heritage Commission. Technical Workshop Series no. 7. Revell G (1994) Guidelines for Managing Plantation Landscapes. Department of Conservation and Land Management, Perth, WA. Revell G (1997) Trees for scenic quality management. In Design Principles for Farm Forestry: A Guide to Assist Farmers to Decide Where to PlantTtrees. (Eds N Abel et al.). Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. Revell G (2000) South-west farm land planning and design project. School of Architecture and Fine Arts, University of Western Australia. Revell G and Cleary J (1998) Landscape Planning Guidelines. Prepared for Department of Conservation and Land Management WA. Revell G, McAuliffe T and Winchcombe J (Eds) (1999) A boree store. School of Architecture and Fine Arts, University of Western Australia, Perth. Riley R (1998) Landscape architecture and education. Landscape Architecture Journal, Spring, University of Wisconsin. Stirzaker R, Vertessy R and Sarre A (2002) ‘Trees, water and salt: an Australian guide to using trees for healthy catchments and productive farms’. Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. Publication no. 01/086.

Stuart-Street A and Revell G (1994) Reading the Remote: Landscape Characters of Western Australia. Department of Conservation and Land Management, Perth, WA. Tonts M, Campbell C and Black A (2001) ‘Socioeconomic impacts of farm forestry’. Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. Publication no. 01/45. Tredinnick M (2004) The genius of the place: an introductory essay. In Sense of Place: A Response to the Environment. (G Seddon). Limited facsimile edition. Bloomings Books, Melbourne. Lamb R (1993) Advancing arguments for the conservation of valued landscapes. In More than Meets the Eye: Identifying and Assessing Aesthetic Value. (Eds J Ramsay and J Paraskevopoulos) pp. 23–36. Australian Heritage Commission, Technical Workshop Series no. 7. Van Pelt JW (1980) Landscape analysis and design for conifer plantations. Journal of Australian Forestry 43(3), 178–188. Weller R (1997) Design with the lot: a takeaway. Unpublished keynote paper presented to W-Edge Conference: National Student Conference in Landscape Architecture. September 23–27, Rottnest Island, WA. Winchcombe J (1999) (Ed.) ‘Southwest farm forestry project. Subproject 3. Farm forestry integration packaging’. National Heritage Trust Farm Forestry Project, Department of CALM Report to DAFF. Winchcombe J (2000) ‘Investigation of the economic and social opportunities for farm forestry in the Dandaragan Shire’. Dandaragan Shire Land Conservation District Committee. NHT Project no. 983017. Winchcombe J and Revell G (2004) ‘Farm forestry and landscape architecture: a feasibility study’. Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation, Canberra. Project no. UWA-68A. Publication no. 04/187. Zube EH, Sell JL and Taylor JG (1982) Landscape perception: research application and theory. Department of Geography, University of Arizona, Tucson, Arizona.

PA R T I I

Productive function of trees in the landscape

9 Wood as a farm product Rowan Reid

Introduction There are thousands of different wood products that farmers may be able to produce, ranging from fine furniture timbers to firewood. Wood is also the basis of many chemically derived products including cellophane, charcoal, dyestuffs, explosives, lacquers, turpentine and yeast. It is possible to identify both virtues and defects for any wood product market and processing method that will affect the value of a tree or forest as a source of raw material. All trees grow wood, but not all wood is worth growing. Although cellulose and lignin are the main constituents of all woods the quality of wood for a particular purpose may be influenced by its colour, density, cell structure, presence of resins, oils or other compounds, distribution of growth rings, grain pattern, natural durability, ease of working or any number of other properties. These vary between species and may also vary with location in the tree, growth rate, tree age, site characteristics and management. The value of a particular tree or log as a source of wood may depend on other factors such as log diameter, straightness, length, knot size or presence of internal growth stresses. Like most other commercial farm products, the forest grower generally receives what’s left after their product has been harvested and transported to the point of sale. Wood is a bulky product so, if the product is of low value or demand is low, harvesting and transport costs can easily amount to more than the sale price, making it unviable to harvest. To increase returns, growers might explore ways of

reducing the costs associated with the production, harvesting and marketing of their timber. Alternatively, there may be ways in which they can increase or add value to their product as it is growing. Distance to market, ease of access, harvest volume, uniformity of growth, tree species, log size and shape, the availability of appropriate skills and equipment, the ability to value-add on site and direct marketing are all important. Every management decision a tree grower makes, from the time of planting through to the final harvest, can influence wood quality, log quality and forest value. It is in every grower’s interest to know what defines quality and value in the wood markets they are interested in supplying. Working backwards from the market specifications to the paddock, growers can balance their views about future market opportunities and prices, non-wood values they’d like their forest to provide, personal preferences and risk, to develop a clear vision of their target forest. Only then can they select the most appropriate species, planting arrangement and management plan for their site. This chapter reviews the nature of wood and how trees and forest grow. Based on market specifications, the following chapters explore the production options for particular timber products including firewood, pulpwood and sawlogs.

Hardwoods and softwoods Trees are classified into two groups – hardwoods and softwoods. Confusingly, the wood of a 145

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hardwood is not necessarily hard. Balsa wood, for example, is actually a hardwood, while some of Australia’s toughest timbers, such as callitris pine, are actually softwoods. The easiest way to distinguish the two is to remember that flowering trees are all hardwoods (eucalypts, wattles, oaks and others) and that cone-bearing trees or conifers are all softwoods (pines, cypresses, spruces etc.). The important difference between hardwoods and softwoods is the cellular structure of the wood. Hardwood timber is made up of four types of cells: small fibres that make up the bulk of the wood structure, large cells called pores or vessels that act as pipes for moving water and nutrients up the tree through the mass of fibres, ray cells that run from the centre of the stem to the bark rather than up and down the stem, which play a role in storing and distributing carbohydrates, and a last cell type that is largely used to store food. With experience, it is possible to distinguish individual hardwood timbers on the basis of the number, size and location of the pores and ray cells. Softwoods have a simpler cell structure. The bulk of the wood is made up of long open fibres which provide strength and transport all the water and nutrients up the stem from the roots. They have ray cells but rarely are these visible to the eye. Although they do not have pores, most softwoods do have large resin canals or ducts. The resin provides an important defence against decay in the living tree.

Tree growth and wood production How trees grow wood

The above-ground part of a tree grows in two ways. Elongation or height growth of the main stem and branches occurs as a result of cell division by the apical meristems (dividing cells) located in the growing tips. Leaf, stem and flower buds arise from the apical meristem and may develop immediately or remain dormant below the bark at that point on the stem. The corky remnant of apical growth is called the pith and can often be seen at the centre of a log. This growth gives the tree height but is not responsible for producing the true wood found in the trunk or branches of a tree. True wood is the result of growth derived from the cambium, a thin layer of cells hidden just below

the bark. The cambium produces both wood and bark and is sandwiched between these two parts of the tree. As a stem thickens the cambium moves out away from the pith and expands to cover the greater surface area of wood. In the living tree, newly created bark cells on the outside of the cambium form the phloem through which the carbohydrates and hormones generated in the leaves flow down the trunk to the root system (Figure 9.1). As new phloem cells are formed older ones dry out, adding to the protective bark. On the inside of the cambium newly fashioned wood cells add to the band of sapwood through which the water and nutrients flow up the tree. The type and number of wood cells produced by the cambium is determined by the concentrations of carbohydrates and growth hormones in the phloem. During favourable conditions for growth high concentrations of carbohydrate and hormones flowing down the phloem result in the formation of the large earlywood cells in the stem. As photosynthetic activity slows during dry or cool conditions (or as the tree enters a dormant phase) the concentration of hormones falls, resulting in small thick-walled latewood cells. These dense latewood cells give rise to the growth rings. In wellwatered temperate regions trees generally produce one growth ring per year. In dry or tropical areas, where growth responds to rainfall rather than temperature, there may be a number of growth rings produced each year, or even none at all. As new sapwood is formed, the inner rings of older sapwood, no longer required for water flow, are retired (the cells being filled with crystals or resins) and become heartwood. The absence of starch and the presence of phenolic compounds often give the heartwood a darker colour and greater natural durability against fungi, insects and other decay agents. Although the process of transition of sapwood to heartwood is not well-understood we do know that there is a relationship between the water needs of the canopy (leaf area) and the width of the sapwood band in most species (Vertessy et al. 1995; Teskey and Sheriff 1996). A large open grown tree with a large canopy is therefore likely to have a wider sapwood band than a tree of similar size growing in a dense forest. Because of the relationship between canopy size and growth rate, fast-growing trees generally have

9 – Wood as a farm product

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Figure 9.1: Cross-section of the stem showing the different wood types and their function.

a wider sapwood band than slow-growing trees of the same species. Tree growth and wood properties

How a tree grows can affect wood properties. It is important that forest growers understand these relationships and the potential to use silvicultural management to control wood quality. This section reviews some important relationships between tree growth and a range of important wood properties. Wood density

Wood density affects the strength of timber, pulp yields, fuel value and many other important properties. Although the wood of some species is naturally heavier than others it is important to appreciate how density varies within the tree and in response to growth. First, in all species wood density varies across the growth ring depending on the location and density of the latewood and earlywood, the transition between them and the distribution of pores (in hardwoods). If the variation in density between the earlywood and latewood is great then the distance between growth rings may be critical for some applications. Variations in strength, appearance or acoustical qualities across the growth ring may make some fast-grown trees (with wide growth rings) of otherwise valuable species unsuitable for products such as flooring, furniture or musical instruments. It is common to think of fast growth resulting in lower wood density but this is generally not the case. In some hardwood species the large open pores form a ring in the early wood (ring-porous species) at the start of the growing season, accentuating the appearance of the growth ring (Figure 9.2). Because

Figure 9.2: (a) Ring-porous oak (Quercus robur). (b) Diffuse-porous merbau (Intsia spp.).

the bands of pores have the same width, irrespective of growth rate, the densest timber actually comes from the fastest-growing trees because there are fewer rings of open pores (Haygreen and Bowyer 1989). This is the case for many of the oak species (Quercus spp.), with the heaviest and most durable timber produced from the fastest-growing trees on the most fertile sites. For barrels, coopers may prefer the lighter slow-grown Oak because it is easier to bend and imparts more flavour to the wine. Australian hardwoods, such as eucalypts and acacias, are usually diffuse-porous. The distribution of pores stays relatively uniform across the growth ring, meaning that growth rate does not directly influence wood density (DeBell et al. 2001). However, research suggests that the average wood density of each successive growth ring in young eucalypts increases as the trees grow and that the highest wood density is found just below the bark (Wilkes 1984; DeBell et al. 2001). This phenomenon is related to the concept of juvenile wood or crown wood rather than growth rate (see below). Wood density may also vary with site location. Many pine species, for example, produce more latewood, and therefore have higher density, when grown closer to the equator. In New Zealand the wood density of radiata pine is classified in some markets by the region in which it was grown: high density (Northland, Auckland), medium density

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(central North Island) and low density (South Island) (Cown et al. 1991). Similarly, in Australia the wood density of subtropical exotic pines of the same age can increase by as much as 10% between northern New South Wales and north Queensland (Harding and Copley 2000). Once formed, the density of wood fibres cannot change. However, the process of tylosis which occurs during the transformation of sapwood into heartwood can increase wood density. Wilkes (1984) examined the basic density (oven-dried) and extractive content of fast- and slow-growing coppice regrowth of six dry sclerophyll eucalypts. He found that heartwood development increased wood density by as much as 20% (from 0.72 to 0.90 g/cm3 in E. sideroxylon) and that faster-grown stems had a higher extractive content than slowergrowing stems. Juvenile wood (crown wood)

In Pinus radiata the density of the timber laid down by the cambium during the first 10 years of growth (the central 10 growth rings in a log) may be as much as 40% lower in wood density than the wood in growth rings laid down after, say, 30 years (Lavery 1986). Because of this many pine sawmillers specify that they prefer logs from forests over 25 years old, due to concerns about the strength of young timber. The industry refers to this inferior timber as ‘juvenile wood’. The average density of the eucalypt timber in successive growth rings also increases with tree age (Downes et al. 1997). However, the fact that the timber of many mature eucalypts is much heavier than most internationally recognised cabinet species, such as Australian blackwood and English oak, suggests that a reduction in wood density could actually be an advantage for some applications. Nonetheless, the high proportion of juvenile wood in a young tree may affect its value where density is important, such as for firewood, durable posts or structural beams. Across a range of species, juvenile wood tends to shrink more due to the greater angle of the microfibres that make up the cell walls of each wood cell and is softer due to the lower proportion of latewood (Dean and Baldwin 1996). The reason why the density of newly formed wood in the trunk of a young tree is lower than that produced in later years is not directly related to age

Figure 9.3: Wood density (oven dry weight/air dry volume) across the width of three 135 mm quartersawn boards sawn from 16-year-old Eucalyptus nitens and a similar board cut from a 65-year-old native forest E. nitens log of similar size. The results show the expected trend of increasing wood density with greater distance from the centre of the log and no clear effect of growth rate or age.

at all. The difference is the relative distance of the newly formed wood cells from the active canopy. In a short tree the lower stem is closer to the photosynthetically active leaves so the concentration of hormones (auxins) and sugars in the phloem will be higher than at the same height in a tall tree (Dean and Baldwin 1996; Jagels 2006). In a timberproducing forest the distance between the lower stem and the active canopy increases over time as tree height, branch length and height to the green canopy increases. This explains the trend of increasing wood density across wide quartersawn boards (Figure 9.3). ‘Crown wood’ is a more accurate term than ‘juvenile wood’, because it develops in areas close to the crown where these concentrations are greater. Natural durability and colour

Many timbers are valued for their natural resistance to decay or insect attack. The sapwood of almost all species is not durable because the cells are open to moisture and contain life-supporting sugars and starches (Bootle 1983). Natural durability and resistance to insect attack of the heartwood of many species result from extractives that fill the cells and impregnate the cell walls during heartwood formation. The tree’s ability to access or produce the chemicals and minerals that provide


resistance to decay or insect attack varies with site conditions and growth rates. Jagels (2006) suggests that soil pH and cation exchange capacity are important in that they may influence the availability of important chemicals. He cautions that high site quality, fertilisation, irrigation or the promotion of rapid growth by thinning may result in a dilution effect which could lead to formation of heartwood with lower natural durability. Wood colour may be correlated with natural durability but is also important in its own right. The colour of freshly sawn wood and how it responds over time varies within a species and may be affected by site quality, growth rate and silvicultural management, but the relationships are poorly understood or inconsistent. Colour variation in Australian blackwood has been studied in Tasmania, New Zealand and South Africa and the findings provide some insight into the complexity of colour variation in timber. Bradbury (2006) found that although darkness was correlated with wood density in a Tasmanian trial, increasing growth rate was not shown to influence either. In New Zealand, Nicholas et al. (2006) found that colour varied enormously across seedlots and sites but was not influenced by stocking rate. However, their results did not support earlier South African research that had suggested blackwood colour was darker in trees growing on well-watered sites with deep organic soils (Harrison 1975). Reaction wood and growth stresses

Compression and tension wood (collectively known as reaction wood) are abnormal wood cells that occur in softwoods and hardwoods respectively. The presence of reaction wood can have a dramatic effect on milling and drying behaviour and wood properties. In eucalypts, tension wood is more prone to collapse during drying and tends not to recover when reconditioned (Washusen et al. 2002). The excessive shrinkage common in tension wood can lead to extraordinary twisting and warping during drying (Washusen et al. 2000). Both tension wood and compression wood can form in response to the tree leaning over or simply flexing in the wind (Washusen 2002). A leaning softwood will form compression wood on the underside of the stem whereas tension wood forms on the upper side of a leaning hardwood (Jagels

2006). Branches naturally have lots of reaction wood to hold them in position, which is why they are rarely used for solid timber. Many people working with eucalypts have experienced end splitting in recently harvested logs or witnessed the bending of freshly sawn timber as it is milled. This is caused by a difference in tension between the outer growth rings and the log core. The development of growth stresses is natural and easily understood if we think of the standing tree trunk as a flexible, rather than a ridged, pole. As new growth rings are laid down around the standing tree, a longitudinal (up the tree) tensile stress is imposed as the cells mature. The result is that the trunk’s outer surface is in tension (like an elastic band) and balanced by compression stresses (like a compressed spring) in the central core. Unlike a ridged pole, the tension near the surface allows the tree to absorb wind stress without breaking. In effect, it is similar to a concrete pole with steel reinforcing rods embedded around the perimeter. The tension is induced during wood cell maturation. Secondary wall thicknessing of newly formed wood cells causes the layer of wood cells to contract. Imagine a cardboard box wrapped in shrinkwrap plastic. The plastic is first loosely wrapped around the carton and is neither in tension or compression. When treated, the plastic shrinks tight around the box, imposing an equal and opposite compression stress on the box. If the box is repeatedly wrapped and the same degree of tension applied to each layer of plastic, the box may eventually crumble. This can also occur in trees, and is seen as ‘brittle heart’ in the centre of the log. If a log with severe natural growth stresses is sawn down the centre each half immediately bows out as the tension wood is able to relax. This creates problems during milling and may require special equipment or sawing patterns (Washusen et al. 2000). Back-sawn boards (Figure 9.4) will bow as a result of longitudinal stresses; quarter-sawn boards will spring. Bow is of less concern as the boards can be easily straightened, so mills may prefer to backsaw highly reactive logs. Spring is difficult to correct without large reductions in recovery. Research supports the observation that growth stresses are less severe in larger diameter logs from short straight trees (Yang and Waugh 2001). There are two reasons suggested for this. First, although

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Figure 9.4: The terms quarter-sawn and back-sawn refer to the alignment of growth rings in the sawn board.

the difference between the tension in the outside of the debarked log and the pith compression in a large-diameter log may be similar, the gradient of change is greater in a small log, causing it to bend more (Figure 9.5) (Yang and Waugh 2001). Second, in tall skinny plantation trees, growth stresses are thought to be a result of bending stresses induced by or in response to wind (Washusen 2002). In one study of straight plantation-grown E. globulus, tension wood was most pronounced at the base of the tree on the side facing the prevailing winds and extended further up the stem in tall slender trees (Washusen 2002). Sturdy, short fat trees are less prone to wind-induced growth stresses. Leaning eucalypt trees can have both growth stresses and bands of tension wood, making them difficult to mill and diabolical to dry.

Figure 9.6: Knotty (left) and clearwood pine sawn from pruned plantations in Tasmania.

Knots or branch stubs

Knots or branch stubs can be a major defect in sawn timber (Figure 9.6). Large, loose or dead knots can greatly reduce the strength of sawn timber and can be unsightly in appearance-grade products. In this respect it is important to distinguish between structural and appearance grades. Structural timbers are graded on the basis of their suitability to carry loads. For a given species, visual grading rules specify the size, location, number and type of knots permissible for each structural grade. Increasingly, mills are using mechanical stress-grading machines to test the bending strength of each piece and grade them accordingly. It is possible that other factors, such as wood density and grain orientation, may compensate for large or numerous knots. However, knots of any size can affect wood values in appearance applications. ‘Clearwood’, timber that is clear of knots or other defects, is usually considered the highestquality and valuable timber grade for use in furniture, flooring, lining boards, joinery or other applications. There are a number of strategies growers can use to control the type, size and location of knots in the log, including stem-pruning and managing competition in the stand. These are discussed in detail in Chapter 10. Tree form and log shape

Figure 9.5: Radial splitting in this small-diameter eucalypt log illustrates the difficulty of sawing quartersawn boards from small-diameter logs.

For sawn timber production, it is often critical that the logs come from straight upright trees. In addition to forming reaction wood and growth stresses,


leaning trees also tend to develop a sweep that makes it diffult to mill long straight lengths. Sharp bends and kinks are also a problem. Although small bends in young trees may grow out over time, the wandering pith that results will lower recovery of the better-sawn timber grades. The core of the log is commonly boxed-out and discarded because of problems of drying boards containing pith. Multi-stems and large forked trees have little or no value for sawlogs. They may be undesirable even in pulpwood and firewood plantations. A tree with two stems does not necessarily have a greater volume than if it had a single stem, since in both cases the effective canopy area is the same. The smaller diameter of the multi-stems increases harvesting costs and the area of bark that has to be removed, and may be more difficult to mechanically harvest. The degree of taper can be critical (Figure 9.7). Where the diameter decreases rapidly up the stem the recovery of sawn timber will be greatly reduced. Logs that are near-perfect cylinders are clearly preferable for most applications. If the taper is excessive, sawmills may try to purchase logs on the basis of the volume calculated using the small-end diameter rather than the mid-point or average diameter. Taper can be influenced by silvicultural management, as discussed in later chapters.

recovery, the lower the fixed costs of milling and the greater the proportion of higher-value grades. Larger logs can be more easily quarter-sawn, may be less prone to problems due to growth stresses and will generally have a lower proportion of juvenile wood. For products in which the sapwood is undesirable, such as species with durable or attractive heartwood, large-diameter logs are essential. Naturally, larger-dimension boards can only be cut from large-diameter logs. For veneers, the larger the diameter the higher the value. Logs of 50–70 cm in diameter (underbark) are often preferred in mills that saw large-section appearance-grade timber. Some of the high-production softwood mills focused on producing structure pine are geared for rapid throughput and may not be able to handle logs over about 55 cm in

Log length Depending on the sawing equipment and product options, a sawmill operator should be able to specify a minimum and maximum log length. The tendency is to prefer longer logs as these will incur lower fixed costs in the harvesting, transporting, milling and drying. However, few modern sawmills can cater for logs greater than 6.5 m. In some cases sawmillers may cut longer logs in two if there is concern about severe growth stresses or taper. Log diameter Whatever the product, harvesting, debarking and loading costs per cubic metre of wood increase with decreasing log diameter, especially if the trees are manually felled. Once cut into lengths, it is the diameter of the log, not the height of the tree, which will be most important to the miller. Generally, the larger the log diameter the greater the sawn timber

Figure 9.7: This pruned spotted gum (Corymbia maculata) has a reasonable diameter but a degree of taper. If left to grow for a few more years, the taper will become less pronounced.

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Figure 9.8: Fiddleback blackwood (Acacia melanoxylon) is prized for guitar backs and other high-value end uses.

diameter. Post and pole markets have strict diameter length and taper requirements. For vineyard posts, the price per cubic metre for small-diameter posts of high strength can be higher than for largediameter posts. For electricity or building poles, an even diameter along a long log length is required. Other defects and effects

Other factors that may affect log quality are rot, insect tunnels, unusual grain patterns, fungal infection, kino veins, resin pockets and fire scars. Although some may enhance the appearance of the timber for use in crafts or designer furniture, defects rarely enhance log value. Where it is possible to see that a particular log has an interesting grain pattern, such as fiddleback or birdseye, it may be worth separating it for sale to a wood worker who can take advantage of its unique qualities. Any foreign material imbedded in the tree, such as fencing wire, electrical insulators, horseshoes or nails, present a great risk to safety and milling equipment. If there is any risk of metal or other solid material in a log a miller will be very reluctant to purchase or saw the timber. In some cases metal detectors are used to scan the log before milling.

Log value and prices Logs are sold in various ways. If the buyer purchases the standing trees they are said to pay a

stumpage price. This is usually an agreed price per cubic metre for different log grades. For example, a farmer might be paid $50/m3 of sawlog and $10/ green tonne for the rest of the tree, which might be used for firewood. The volumes might be assessed on the basis of truck weighing-station measurements. When purchasing standing timber, the buyer covers the costs of harvesting and transport. If the forest owner fells the trees and delivers them to the mill, or employs a contractor to do so, the point of sale is at the mill rather than at the stump. Sawlogs might be delivered to one buyer and pulpwood to another. The price received for each product is called the mill door price. Other ways to sell timber include selling the land and timber as a package, selling the harvesting rights as a lump sum, sorting the logs into grades and selling at the farm gate, and value-adding by processing and/or drying then selling sawn timber, firewood, posts or other products directly to wholesalers and users. Irrespective of marketing method, the value of a grower’s logs will reflect the availability of processing facilities, competition in the market, demand for the final product, the volume available for sale, ease of access and many other factors. Small growers commonly find they have insufficient volume or market leverage to negotiate a satisfactory price, especially when there are large producers in the same area providing a very similar product to a small number of buyers. It is therefore important that small growers explore ways of improving their product quality, targeting specialist markets or value-adding. Figure 9.9 shows the price-quality curves for a range of log types sold by the Victorian government in 1995. In Australia, exotic plantation softwood logs of similar wood characteristics have traditionally been sold on the basis of diameter, with the larger logs receiving higher prices per cubic metre. As the hardwood timber industry becomes more sophisticated and the quality of logs from native forests declines, the price differential between high- and low-value eucalypt logs appears to be increasing. In some cases there are minimum specifications that, if not met, mean the logs have no value. For example, small-diameter cabinet timbers such as Australian blackwood are often not worth milling and have no woodchip market due to their colour.


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Figure 9.9: The price-quality curve for different logs based on Victorian government stumpages in 1995.

It is important that growers appreciate that many factors can influence quality. With increasing world demand for plantation-grown eucalypt pulpwood and improved technology for milling small-diameter logs, value may relate more to the economies of scale than to log dimensions. Whatever the determining factors, the steeper the gradient of the pricequality curve the greater the reward for growers who can produce higher quality. The greater demand for high-quality logs makes it easier to attract buyer interest in small lots and reduces the impact of transport distance on stumpage prices. All too often, forest owners discover that their timber plantations or native forests are unviable to harvest due to the low value of the trees and the high costs of access, harvesting, loading and transport. The economics of harvesting dictate that where the logs are low in value, the operation must be highly mechanised to be viable. The equipment required for mechanised logging is very expensive and the contractors involved commonly prefer large forest areas with relatively easy access (Figure 9.10). Manual falling with chainsaws and extracting logs with tractors is usually only viable when log value is high (Figure 9.11). Research in Australian pine plantations suggests this system is only competitive in well-spaced pruned stands of largediameter sawlogs (>40 cm) (Reed 2001). One advantage of pruning is that it greatly reduces the labour required to delimb. Wider tree spacing at harvest also makes falling and access easier. Transport costs can be as high as 10c/km/t, thereby reducing the payment to the grower by

more than $10/m3 for logs that need to be hauled over 100 km. Growers located a long way from potential markets should consider high-value logs and partial value-adding. Value-adding may also increase the number of potential buyers and ultimately the equivalent stumpage price, particularly in areas where there are very few local processors and therefore limited competition. The choice of site, tree species, silviculture and even harvesting method should be based on a defined product preference. It is important that these also reflect the growers’ interest in non-timber values such as shelter for stock, biodiversity, aesthetics or land degradation control and their attitude to the inherent risks associated with any forestry investment. Target tree specifications may include the species, tree and log dimensions such as diameter, length or form, branching habit and preferable harvest age. At the forest level there may be specifications related to the minimum harvest volume, preferred site attributes and access. For example, for pulpwood production a minimum harvest volume may be 1000 t. If a grower produces less it may not be possible to attract a harvesting contractor, irrespective of tree quality. Small growers, or those who do not intend to clearfell large areas, generally need to focus on producing trees of high value in order to overcome the higher management and marketing costs. Pruning, for example, is more commonly adopted by small growers because it adds value to the forest, reduces harvesting costs and may overcome the need to find markets for low-quality or small-diameter logs.

Figure 9.10: Mechanical harvesting is efficient and safe but contractors usually require large volumes and good access.

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Tree growth

Tree shape, height growth, diameter increment, branch development and ultimately wood quality are defined by a combination of the apical meristem growth, which occurs at the tip of the leading stem and all the branches, and the cambial growth, which leads to trunk and branch thickening. Although both growth types may occur simultaneously on the same tree, what initiates each growth type, their pattern of development and how they respond to various growing environments are very different. Of interest to forest growers is the influence of apical meristem growth on tree height and form and the influence of cambial growth on stem and branch diameter. Apical growth: tree height and stem form Figure 9.11: Manual falling can be efficient in mature widely spaced and pruned plantations, but requires special skills.

Product specifications are likely to vary over time so it is worth considering alternative market opportunities. The common recommendation is to aim for a high-quality forest that meets the criteria of a number of possible markets. A perfect hardwood sawlog can always be used for firewood, but a perfect firewood plantation is unlikely to produce high-value furniture timber.

Principles of silviculture Trees and forests are, by their very nature, dynamic – they grow and decay. It is not always possible to hasten or slow growth, but it is possible to set and steer it in different directions. The term ‘silviculture’ (from the Latin ‘silva’, meaning wood) refers to the practice of purposefully manipulating forest composition and growth. Although past emphasis was on wood production, the term is now used widely to refer to managing forests to enhance their value for biodiversity, water production or any other purpose. The ability to direct tree growth is based on an understanding of how trees grow, how their growth responds to particular environmental conditions, and the potential to intervene by pruning, thinning or other management options.

The rate and extent of height growth in most tree species grown in Australia is governed by environmental factors such as soil structure, water availability, fertility, temperature and humidity. Height growth begins rapidly at the start of the growing season and continues until water availability or humidity decline. This growth pattern is different from some northern hemisphere temperate hardwoods species, which have a pattern of rapid early height growth at the start of the season which suddenly stops after a period determined by conditions in the previous growing season when the buds were formed (Shepherd 1986). When grown in Australia, these predeterminant growth species tend to show slower overall height growth because they can be retarded by late-season droughts and are less able to take full advantage of good growing conditions. Exposure to damaging or drying winds or excessive solar radiation can stunt height growth. Figure 9.12 shows the average height growth of young flooded gum (Eucalyptus grandis) growing on a flat exposed irrigation farm in northern Victoria. The plantation was established as a variable spacing trial covering a range of stocking rates from 1000 stems/ha had no further impact on average height growth. It is demonstrated across a wide range of species and sites that crowding does not force trees to grow taller. Crowding does, however, reduce canopy size, suggesting that height growth is driven by the photosynthetic activity occurring within or very close to the leading shoot and not by the amount or the vigour of the lower canopy. If an individual tree is unable to keep up with its neighbours the leading shoot will be shaded, stalling height growth. Overtopped trees quickly become suppressed and, depending on their shade tolerance, may succumb. Self-thinning is common in the sun-loving eucalypts whereas many rainforest species can persist below the main canopy, awaiting an opportunity should a gap be created above them. For timber production, straight singlestemmed trees are preferred. If growing on a stable soil in a well-sheltered location, forest trees tend to grow straight unless the light distribution is uneven and the species has a tendency to bend towards the light (phototropism). When growing

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Figure 9.12: Height growth of E. grandis in a spacing trial at Kyabram (Vic.) over a five-year period, showing how exposure at lower stocking rates retards height growth (data provided by John Kellas).

phototropic species (Eucalyptus, Acacia, Pinus etc.), it is important to avoid sites with heavy shading on one side as this will cause the trees to bend. In these situations it is better to grow geotropic species, like the Australian southern silky oak (Grevillia robusta) and many introduced species from higher latitudes such as English oak (Quercus robur) that tend towards vertical growth irrespective of the light distribution. The ultimate height which a tree will reach on a particular site is largely predetermined by the soil and climatic environment in which it is growing and the potential of that species to lift water from the soil to the uppermost leaves. The tallest trees in the world are the mountain ash (Eucalyptus regnans) of south-east Australia and the coast redwood (Sequoia sempervirens) of the coastal forests of North America. Both species can attain heights of over 100 m when grown on deep, moist soils in well-sheltered high-rainfall areas. However, if soil moisture is limited during the growing season or the leading shoots are repeatedly exposed to low-humidity air, the trees’ ability to sustain water supply to the leading shoots is compromised and height growth is retarded (Salisbury and Ross 1992). The difficulty in maintaining a continuous water supply to the leading shoots explains why tree height growth slows early on very harsh sites. Figure 9.13 shows modelled height growth curves for Eucalyptus globulus growing on a range of sites. Over time the plantations approach a maximum height for that species which reflects the productivity of the site. If there are no serious nutrient deficiencies, then soil depth, texture and waterholding capacity, along with rainfall, temperature and humidity, seem to be the most important factors governing site productivity. Because height growth is largely independent of stocking rate and difficult to change, the height of a plantation of a particular species at age 10 or 20 is commonly used as index of inherent site quality. On marginal sites, where the top height is reached at an early age, it is common to find an even-aged plantation or regrowth forest of droughttolerant eucalypts that has become ‘locked up’. Height, diameter and therefore volume growth appear to have stopped. Unless the dominant trees can grow tall enough to shade their neighbours there will not be the self-thinning of suppressed

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Figure 9.13: Modelled height growth of E. globulus in Gippsland and south-west Western Australia (reprinted from Wong et al. 2000). Each pair of lines represents a different site-quality class.

trees that is required to release more growing space. Growth of the dominant trees can only resume if gaps are created in the canopy by storms, fire, disease or artificial thinning. This introduces another important relationship between height growth and tree form. If a young tree is putting on rapid height growth the lower branches, even in open-grown trees, tend to be smaller, resulting in a more conical form. When height growth slows, due to exposure or site limitations, the canopy tends to round out and the lower branches become larger. Once established, height growth can be maximised by ensuring the leading shoot is sheltered from dry or damaging winds. Increasing the stocking rate above that required to provide mutual shelter will not encourage the dominant trees to grow any taller. Increasing competition will, however, affect branch development and diameter growth which occurs at the cambium. Cambial growth: tree diameter and branch size

The cambium is a thin layer of dividing cells located just under the bark, that produces new wood cells on the inside and bark cells on the outside. Initially the bark cells act as the phloem transporting sugars and hormones down the stem from the leaves before dying and providing protection for the cambium. The new wood cells become active sapwood, transporting water and nutrients up the stem. Both the wood and bark growth contribute to increased stem diameter,

Figure 9.14: Results of a tree spacing experiment involving E. pilularis measured at 11 years of age (data from Borough et al. 1984).

although shedding of the outer bark means the increase in bark thickness is less significant. Unlike height, tree diameters do not approach a maximum for the site. Theoretically, trees can continue to put on diameter indefinitely although they will eventually succumb to disease, drought or old age. However, the amount of wood produced by the cambium is much more sensitive to competition than is height growth (Kozlowski et al. 1991) because it is driven by total leaf area and vigour above that point on the stem or branch. The rate of diameter growth at any point simply reflects the concentration of carbohydrates flowing down through the inner bark. Once site resources (particularly light and moisture) become limiting, any increase in competition will lead to a direct reduction in the size or efficiency of individual tree canopies. As a result, the amount of carbohydrates produced by the leaves and fed down the branches and trunk for cambium growth will be reduced. This results in reduced diameter growth on individual trees. The differences in diameter growth of trees of the same age growing on the same site can be startling. For example, the results of a spacing trial of E. pilularis (Figure 9.14) show that the widely spaced trees had a mean diameter almost twice that of trees in the dense plantation after just 10 years. The impact of competition on diameter growth is best shown by examining the annual diameter increments (Figure 9.15). Using forest basal area as a measure of competition, it is evident that diameter growth in trees grown in open plantations can


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Figure 9.15: Mean diameter growth of E. grandis in a four-year-old spacing trial (data from Ryan 1993).

be four or five times that of the same species growing in a dense forest (Figure 9.16). Basal area as a measure for competition

The level of competition is related to the total leaf area of the trees that make up the forest canopy. Unfortunately, it is difficult to measure total leaf area. We do know that the area of sapwood in a tree is directly related to the volume and health of its canopy (Langstrom and Hellqvist 1991). This is not surprising, given that one of sapwood’s roles is to transport water to the canopy. The more canopy, the greater the area of sapwood required. Everything else being equal, if there are two trees of the same diameter the one with the more active canopy will have a wider sapwood band (Figure 9.17).

Figure 9.17: The width of the sapwood band reflects the vigour of the tree canopy. The teak tree (Tectona grandis) on the left was from a 63-year-old high-stocked plantation in northern Thailand, and the other was from a widely spaced 25-year-old plantation in the same area.

Basal area is a measure of the total cross-sectional area of the tree and includes both heartwood and sapwood. It is therefore incorrect to assume that a total basal area of 10 m2 /ha implies the same level of competition in a plantation of small trees as in a plantation of larger trees. In order to maximise diameter growth in a eucalypt plantation, the first thinning may reduce basal area to less than 5 m2 /ha, the second thinning to 10 m2 /ha and the third to 15 m2 /ha. The difference is the result of the increasing area of heartwood in the trees as they grow. Competition and volume production

The total volume of timber in an even-aged plantation is directly proportional to the forest basal area and tree height. If, for example, all the trees in a forest were the same height and had perfectly conical form, the total volume of tree stem per hectare can be easily calculated: Volume (m 3/ha) = basal area (m 2/ha) # height (m) ' 3

Figure 9.16: Diameter growth is driven by the leaf area, so wider spacing promotes faster diameter growth. This Sydney blue gum (E. saligna) is less than 20 years old (photo provided by D. Jenkins).

Once mutual shelter has been achieved, little can be done to enhance height growth. Basal area, on the other hand, increases with increasing tree stocking and average tree diameter. If the object is to maximise the volume of timber, it is desirable to maintain a high stocking rate (Figure 9.14). This is why pulpwood plantations are established at >1000 stems/ha (. Kozlowski TT, Kramer PJ and Pallardy SG (1991) The Physiological Ecology of Wood Plants. Academic Press, San Diego. Langstrom B and Hellqvist C (1991) Effects of different pruning regimes on growth and sapwood area of Scots pine. Forest Ecology and Management 44, 239–254. Lavery PB (1986) ‘Plantation forestry with Pinus radiata: review papers’. School of Forestry Review Paper no. 12. University of Canterbury Press, Christchurch. Nicholas I, Dungey H, Gifford H, Cox J, Hodgkiss P and Jones T (2006) Preliminary analysis of the wood properties of Acacia melanoxylon R. Br. Thinnings from five North Island regime trials in New Zealand. In Acacia Utilization and Management: Adding Value. Australian Blackwood Industry Group: 4th Blackwood Workshop. Rural Industries Research and Development Corporation,Victoria. Reed J (2001) Scale economics in farm forestry with special reference to timber harvesting. Master of Forest Science thesis. School of

Resource Management, Dept of Forestry, University of Melbourne. Ryan P (1993) Hardwood plantations in Queensland? Why not! Outline of a seminar presented at Gympie, Qld, 13 August 1993. Unpublished. Salisbury FB and Ross CW (1992) Plant Physiology. 4th edn. Wadsworth Publishing, California. Shephed KR (1986) Plantation Silviculture. Martinus Nijhoff, Netherlands. Teskey RO and Sheriff DW (1996) Water use by Pinus radiata in a plantation. Tree Physiology 16, 273–279. Vertessy RA, Benyon RG, O’Sullivan SK and Gribben PR (1995) Relationship between stem diameter, sapwood area and transpiration in a young mountain ash forest. Tree Physiology 15, 559–567. Washusen R (2002) Tension wood occurrence in Eucalyptus globulus Labill. II. The spatial distribution of tension wood and its association with stem form. Australian Forestry 65(2), 127–134. Washusen R, Blakemore P, Northway R, Vinden P and Waugh G (2000) Recovery of dried appearance grade timber from Eucalyptus globulus Labill. grown in plantations in medium rainfall areas of the southern Murray-Darling Basin. Australian Forestry 63, 195–201. Washusen R, Ades P and Vinden P (2002) Tension wood occurrence in Eucalyptus globulus Labill. I. The spatial distribution of tension wood in one 11-year-old tree. Australian Forestry 65(2), 120–126. Wilkes J (1984) The influence of rate of growth on the density and heartwood extractives content of eucalypt species. Wood Science and Technology 18, 113–120. Wong J, Baker T, Duncan M, McGuire D and Bulman P (2000) ‘Forecasting growth of key agroforestry species in south-eastern Australia’. Joint Venture Agroforestry Program, RIRDC/LWRRDC/FWPRDC. RIRDC Publication no. 00/68. Yang J and Waugh G (2001) Growth stress: its measurement and effects. Australian Forestry 64, 127–135.

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10 Growing high-quality sawlogs Rowan Reid

This chapter explores tree and forest growth in greater detail, building on the outline provided in Chapter 9, and examines the application of silvicultural techniques such as pruning and thinning as tools to enhance growth rate or log quality.

Target logs for sawn timbers and veneers At the mill the value of a particular log will reflect the volume, quality and value of the products that can be extracted by the miller. This depends not only on the characteristics of the log but also on the equipment being used, the skills of the operators, the available markets and the scarcity of alternative raw material. Generally, for logs of a suitable species and length, the larger the diameter and fewer the defects the greater the value. As well as having lower fixed production costs per cubic metre, larger-diameter logs generally yield a higher recovery of higher-grade products such as wide boards. Recovery rate refers to the volume of marketable sawn timber that can be extracted from the round log, expressed as a percentage of total log volume. In softwoods, such as pine, recovery rates are often as high as 50–60%; for hardwoods, such as eucalypts, they are generally much lower. This reflects differences in the inherent wood characteristics, sawing patterns and market opportunities. With most sawn hardwood now going into higher-quality appearance-grade markets such as flooring and

joinery, the emphasis is more on quality and value recovery than simply on volume. Large defect-free logs allow millers to employ selective sawing patterns to avoid defects, improve product performance and recover large-section sizes that have higher value (Figure 10.1). What constitutes a defect depends on the market. If clean, strong, consistent and attractive timber is required, the following are commonly considered defects that will downgrade the value of a piece of sawn timber or veneer:

s s s s s s s

branch knots; rot or holes resulting from fungi and insect attack; presence of the pith; presence of sapwood; surface or internal checking; wane – incomplete cross-section size due to the edge of the log; twisting, spring, bow or cupping.

There are also factors that may increase value, such as:

s s s s s

perfectly straight grain along the whole length; even or desired timber colour; attractive grain patterns such as fiddleback or birdseye; close or regular distribution of growth rings; perfectly quarter-sawn or back-sawn boards (see Figure 9.4, p. 150).

161

162


There is much the forest owner can do to enhance the value of their trees for sawlog and veneer production through silvicultural management.

Rotary-peeled veneer has the grain pattern of perfectly back-sawn timber. Clean sheets are preferred for the outer layers of the plywood sheet especially when it is to be used for panelling, doors or concrete formwork. In some cases a different species is used for the outer layer. Large-diameter logs provide the best yield, not only because of the cost of setting each log on the lathe but also because the central core is often discarded. The alternative method of producing veneers is by slicing thin sheets off a sawn block. A 250 mm wide board is first sawn from the log in a conventional sawmill. The block is then passed though a large slicing machine which removes a thin sheet of veneer the same width as the original board. This can be dried immediately and glued onto a panel board, creating the appearance of a solid piece of timber. Sliced veneer of high-value native timbers like Australian blackwood is commonly used in commercial cabinet work. Only the most valuable or attractive timbers are sliced. The logs must be large enough for the production of wide boards suitable for slicing.

What is a veneer log?

Growing clean, straight large-diameter logs

Veneers are thin sheets of solid timber that are used in the production of plywood and panels. There are two ways in which veneers are produced from a log. For plywood production, which requires large sheets of veneer, the log is peeled on a rotating lathe. A large log can produce tens of metres of continuous veneer which is then cut to size, dried and layered to form a 3, 5 or 7 ply sheet. Each layer of veneer in the plywood is aligned in the opposite direction, giving plywood its strength.

You don’t need to be a sawmiller to be able to recognise a potentially perfect sawlog or veneer log tree: a tall straight bole of large diameter clear of branches, rising to a well-balanced canopy. However, you may need the eye and the ear of an experienced bushworker to be able to judge the internal qualities of a mature native tree since even the bestlooking trees may carry internal rot arising from fire, disease, insect attack or wind damage. Even in a planted forest it is difficult to know the internal qualities of the log. The outside of the tree may be clear of branches and there may be no indication of how large the tree or branches were when they were consumed by the growing stem. For this reason it is important to document the history of the forest, even to the point of taking regular photographs and measurements, to be assured of the quality of the trees. This is especially important if a premium for pruning is expected. What distinguishes the production of sawlogs from other forms of wood production is an emphasis on diameter growth, branch development and wood quality rather than simply volume. This chapter begins by looking at what we can learn

Figure 10.1: The wholesale value per cubic metre for 25 mm thick kiln-dried Victorian ash, illustrating the impact of board width and quality on value. Wide boards can only be sawn from large-diameter logs (data based on 2005 prices provided by a Victorian sawmill).

Figure 10.2: Sliced veneers of Australian blackwood (Acacia melanoxylon) show a range of colour and grain patterns.

10 – Growing high-quality sawlogs

Figure 10.3: High-quality solid timber products generally require large-diameter logs. Richard Moore stands in front of a 16-year-old Eucalyptus globulus with a kitchen cabinet door panel made from an earlier harvest of pruned trees from the same site.

from natural forests about how trees might grow in planted forests. The discussion is focused on eucalypts, with passing reference to other species grown for sawlogs on Australian farms. As a group, the eucalypts provide an ideal basis for discussing growing high-quality timber. Not only are eucalypt species suited to almost every farming landscape in Australia, but they also provide examples of almost every issue and challenge faced by sawlog growers.

Learning from nature: how native forests grow wood With the vast majority of eucalypt sawlogs sourced from native forests, it is worth reviewing native eucalypt forest growth patterns and how they influence log characteristics and wood quality. Understanding how trees grow in a native forest provides some principles that growers can use to manipulate growth in their own plantations. A forest or plantation is a community of individual trees that each affect the growth and development of their neighbours. The interaction between individual trees has a direct influence on their height

growth, diameter increment, branch development, tree health and productivity. The simplest natural forest systems in Australia are probably the even-aged single species wet sclerophyll forests of mountain ash (E. regnans) and alpine ash (E. delegatensis) in Victoria and Tasmania. These native forests behave in a similar manner to even-aged monoculture plantations, even those growing in much lower-rainfall areas. The dynamics of mixed species multi-aged forests are more complex, but the principles of forest growth in the plantations and even-aged native forests still apply. An intense bushfire in an ash forest in southern Australia kills the mature trees, releasing their seed onto a fertile ash bed. The following spring more than a million seedlings per hectare may begin competing for the available resources. The competition results in winners and losers. The stocking drops quickly: a year later there may be 100 000 surviving seedlings per hectare and 10 years later it could drop to less than 10 000. Figure 10.4 shows the self-thinning that occurred in a natural forest of alpine ash (E. delegatensis). Eucalypts are light-demanding and will succumb to competition when the canopy of neighbouring trees overtops them. From a stocking of >5000 stems/ha (an average spacing of

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